When using virCommandRunAsync and saving the pid for later, it
is useful to be able to reap that pid in the same way that it
would have been auto-reaped by virCommand if we had passed
NULL for the pid argument in the first place.

* src/util/command.c (virPidWait, virPidAbort): New functions,
created from...
(virCommandWait, virCommandAbort): ...bodies of these.
(includes): Drop duplicate <stdlib.h>.  Ensure that our pid_t
assumptions hold.
(virCommandRunAsync): Improve documentation.
* src/util/command.h (virPidWait, virPidAbort): New prototypes.
* src/libvirt_private.syms: Export them.
* docs/internals/command.html.in: Document them.