1. port the linux socket compatibility layer form freebsd
    2. in freebsd, the compatibility is done to support execution of
       32bit ELFs, so I had to rewrite most of the code in linux_socket.c
    3. special note for sendmsg/recvmsg - I put an assert that we don't handle
       ancillary data (msg->msg_control), so calls that are made to these
       functions with msg->msg_control != NULL will fail. if a need emerge
       we can handle this part later. also, the original compatibility layer
       handled only 2 types of messages: credentials and rights,
       which we ignore in OSv.

A pthread created with the attribute
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
Does not need to be pthread_join()ed - and its resources, most importantly
its stack, should be deleted as soon as it is done.

Previously, we ignored this mode, causing its users (Java uses it for all
its threads!) which do not ever pthread_join() on these threads, to leak
the threads' stacks.

This patch adds support for this mode, and adds a test to tst_leak.cc
which verifies that before the patch, we had a leak, and now we don't.

Unfortunately, this patch is a bit ugly, and I was surprised how much time
it took me to actually get it to work :( Because of the convoluted
relationships between pthread_*, pthread_private::pthread, sched::thread
and sched::detached_thread, I ended up, after many rewrites (!) duplicating
the "reaper" code from sched::detached_thread to do something very similar
for detached pthreads. I.e., when a detached pthread finishes, it can't
pthread_join() itself (which would cause a mess as it destroys its own
stack) so instead it tells a different thread, the "reaper" thread, to
run this pthread_join().

In the future it would be nice to create one generic "reaper" which
is used by both detached sched::threads and pthreads, or even find
a way to allow a thread to pthread_join itself as its last action.

This reverts commit efa61687.  Its purpose
was already done by commit 61679ede.

Pointed out by Nadav.

Memory debugging facility.  Compile with conf-memory_debug=1.

This allocator works by giving each allocation its own virtual address
range which is not reused for later allocations.  After a free(), the
range is made inaccessible, forever, so use-after-free will result in a
page fault.

Sub-page overruns are also detected by filling unallocated space with a
pattern, and checking whether the pattern has been altered during free().

(gdb) osv pagetable walk $addr

Early code may enable interrupts and get hit by a spurious interrupt.

If we are preempted in wait_until() while the thread state is waiting, the
scheduler will put us to sleep; then a wake() will arrive, and we'll go back
to sleep in me->wait(), for which no wakeup is guaranteed.

Disable preemption to avoid this case.

yield() has a random preempt_enable in it, drop.

Will be used for the overrun detector.

Easier for most users.

Prepare for an alternate leak-detecting implementation.

0xffff800000000000 is used by Xen, so avoid it.

needed for the implementation of F_DUPFD in fcntl()

the kern_xxx functions are needed by the linux compatibility layer.

needed by linux_socket compatibility layer.

Eliminate duplication.

When memory is badly corrupted, our attempt to print "Aborted" in abort()
can cause an endless recursion of abort()s, filling the stack and eventually
(since we don't have a stack guard) everything. Let's just avoid printing
the "Aborted" message if inside an abort().

In flush_tlb() avoid the ipi and synchronization protocol if there's just one
cpu. We still a tiny amount of memory and a vector for this purpose even if
there's just one cpu (if it really bothers anyone, I can fix this too...).

Happens in some corner cases.

The current implementation returns 1, leading the JRE to optimize for
a uniprocessor host.  Unfortunately those optimizations are unsafe on SMP,
leading to hangs.

Fix by returning the real number of processors.

Sorry, also forgot to commit this earlier!
Add a new function to send API to all processors accept this one.