Improvements to 'ps -p <x>'. If <x> is a process id that does not exist, then
just print the header (if any) and exit, thus matching the behavior on -stable
and other OS's.

Also adds support for <x> being a comma-separated list of processes, and does
a much better checking for invalid-values of <x>, such as 'ps -p someword'.

Reviewed by: mentioned on freebsd-current
MFC after: 10 days