now it downloads a ~200MB file and validating md5 on it.

the old implementation used threads for dispatching callouts, each callout
owned a thread and it suffered from a race where a callout structure could have
been deleted before the callout thread even begun.

the current implementation is dispatching all callouts in a single callout
dispatcher thread, it maintains an ordered list of callouts to achieve that.

this patch solve a crash with the TCPDownloadFile test, that now proceeds.

the mbuf ext buffer is freed in the dtor, so it should be called before finit.
this is fixing a crash that surfaced by using the conf-memory-debug=1

this haven't caused a real bug, I just noticed it while tracing.
it may be dangerous if in some flow, the stack will not be zeroed

strerror_r is needed by the JVM in order to print errors correctly.

A todo item to avoid unnecessary fpu state saving

Things we still need to do to use the lockfree mutex

Following an idea raised during our last discussion on TODOs, I committed
a new directory "todo/", and one file in it, todo/mm, with TODOs and ideas
for memory management.

I suggest that we allow free commits to this directory - with no overhead
of "review" of these informal todo items.

If we decide we don't like this format, we can easily move the content of
these files to some other tool or database or whatever.

Oops, forgot to commit the removal of this redundant line (thanks Dor
for spotting it).

README's instructions on leak detection were out-of-date in several points,
so corrected it.

Fix two errors in IsaSerialConsole::reset(). The visible effect of these
errors was a spurious \001 (^A) character which always appeared at the
beginning of every OSV's run output.

The two errors were:
1. The DLAB bit on the LCR was incorrectly set (a spurious
   "& LCR_DIVISOR_LATCH_ACCESS_BIT" appeared in the code), causing the
   next outb(1) to go to the output register (and output a "1" character)
   instead of setting the baud rate.
2. When writing the FCR register, the "ioport + " base forgotten.

I took the opportunity and added better names for the constants,
removed a couple of duplicate constants, and added comments.

Added a "--noshutdown" option to loader which prevents a shutdown
(and a poweroff), after main() returns.

Note that this doesn't just replace poweroff() by halt() - with this
option, the system *isn't* halted after main() returns, but rather
continues as usual - possibly still running other threads that main()
didn't wait for, running various system threads and services, and so on.

Context switch tests and related optimizations.

Conflicts:
	bootfs.manifest
	build.mak

kvmclock changes always come from the same cpu, so a real memory barrier
is not needed.

Replace with a compiler barrier.

If there's nothing in the cpu_set (which is fairly common), there's no
need to use an atomic operation.

Builds on osv an Linux.

Tests context switch performance:
  - between threads co-located on the same cpu
  - between threads on different cpus
  - between threads placed by the scheduler policy

Avi recently added extern "C" to pwrite64 (after the header change apparently
removed this declaration from the header file). Also do this to pread64 -
otherwise the "derby" benchmark (from SPECjvm2008) cannot run.

The following test currently frequently crashes - with an abort or
assertion failure.

It's a very simple test, where 10 threads do an endless yield() loop.
While yield() itself is not very important - and doesn't even implement
the promise of sched_yield(2) to move the thread to the end of the run
queue - this test failure may be the sign of a scheduler bug that needs to
be fixed.

Allows cross-platform code where the APIs aren't the same.

Call the new poweroff() function after the payload finishes running.
Makes sense on a cloud (why would you want to pay the provider after
your workload is done?) as well as for our benchmarking, where we want
qemu to exit after running the benchmark.

When the "--leak" option is used, instead of calling poweroff(), we
call hang(), so that QEMU continues to run and we can attach the
debugger to run "osv leak show".

Note that before this patch, if the payload spawned threads, they could
continue running after the payload's main() return. This is no longer
the case - after main() returns, the entire virtual machine is shut down
(or just hung). This is reasonable behavior, though: If the payload needs
some threads to continue running, it should join() them before returning.
The behavior on Linux (and Posix threads in general) is identical to our
new behavior: When main() of a multithreaded program returns, all threads
are killed.

abort() did the same thing as the new osv::hang(), so let's just use
osv::hang(). Note that it's important that osv::hang() doesn't print
anything - abort() does, but avoids infinite recursion that can happen
when abort()'s printing itself causes a crash, and another abort().