Facilities for capturing call stack on arbitrary tracepoints.

New command: 'perf callstack <tracepoint>'

Although we are not expecting to have many interfaces, it is still useful
to print the interface name when we fail to set its parameters. (For starters,
we may very well have plenty, and it is also good for debugging)

Replace the lambda-based with_lock API with a block-based API.

The current with_lock() has the downside of using relatively large lambdas.
Lambdas need extra syntax (and thought) to capture variables, and change
the meaning of 'return', 'break', and 'continue'.

The new with_lock (named WITH_LOCK, since it's a macro) accepts a statement
instead of a lambda:

   WITH_LOCK(mtx) {
       do_something();
       WITH_LOCK(preempt_lock) {
          do_something_else();
       }
   }

In a WITH_LOCK context, any return, break, continue, or exception which exits
the block will cause the lock's unlock() method to be called.

  sprintf(fmt, ...) - returns a std::stream
  fprintf(os, fmt, ...) - prints to a std::ostream&

Easier than the usualy method of constructing a temporary ostringstream
and formatting into that.

Use:

  perf callstack tracepoint

to list frequent callstacks for a tracepoint (from 'perf list')

The collector can attach to a tracepoint and collect backtraces; later
the traces can be dumped as a histogram.

Needed for safe backtrace.

A backtrace() implementation which is safe for use in wierd contexts like
interrupts.  Needs -fno-omit-frame-pointer, but doesn't crash if some object
is compiled without it.

Some of our files include headers inside an "extern C"  declaration.  The
problem with that eventually we will have definitions with C linkage - since
they are inside extern "C" - being included in places where __cplusplus is
defined. I specifically worked around this int the sx_xlock implementation,
but it is starting to create bigger problems. Fix it by making all headers
usable from whichever place by themselves, instead of relying in extern inclusions
in the cpp files

Mostly to get the X bits on directories right for lookups, as well
as checks for hard readonly files.

It may not be available yet.  Change the mode checks to use zp->z_mode,
and remove the devvp checks as we do not support device nodes outside
of devfs.

We might not have a vnode assigned to a znode yet when doing access
checks from lookup.

truncate() and ftruncate() aren't wired up in the VFS yet, so far this is the
only way to get to vnop_truncate.