procdesc: Add support for process descriptors

Add support for process descriptors, which are special file descriptors
associated with a process.  The termination of a task that has an extant
process descriptor is reported as a POLLHUP event on the file
descriptor, rather than by generating a SIGCHLD to the parent process.

Also, wildcard wait4(2) operations do not reap zombie processes that
have an extant process descriptor (although a wait4(2) operation that
explicitly specifies the pid of such a process will reap it).

When all process descriptors associated with a process are closed, the
process is terminated with SIGKILL unless the PD_DAEMON flag was set at
process creation time.

Signed-off-by: David Drysdale <drysdale@google.com>
procdesc: wire up process descriptor syscalls

Add the hooks for syscalls related to process descriptors.  These are:
 - pdfork: create new process, as per fork(2), with associated
   process descriptor.
 - pdgetpid: get the PID value associated with a process descriptor.
 - pdkill: send signal to process identified by process descriptor.
 - pdwait4: perform a wait4(2) operation on a process identified by
   process descriptor.

Signed-off-by: David Drysdale <drysdale@google.com>