pgrep: introduce support for namespaces

A PID should be specified with --ns:
	$ pgrep --ns 12345
which will only match the processes which belong to to the same 6
namespaces. It is also possible to specify which namespaces to test:
	$ pgrep --ns 12345 --nslist mnt,net,ipc
which will match processes that belong to the same mount, network and
IPC namespaces as PID 12345.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>