allows the dhcp worker to see incoming dhcp packets using a simple hook.
no reason to make this kind of hook generic (a BPF alternative) as there
are already many protocol specific hooks in the networking stack.

if we ever need to do more hooking, the pfil interface may be used or we
can implement a generic solution, but it's not reasonable to do so for
a single user (dhcp).

Use DHCP to discover an IP address per each interface, DHCP packets are being hooked
in the networking stack in ip_input and queued for deffered processing using a
dhcp worker thread. Sending dhcp packet is done directly over ethrernet (building
IP and UDP).

There's still alot to be done: setting up lease time, timeouts, more error handling
but it's possible to implement these later on.

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.