libprocessgroup: Check validity of uid and pid arguments

A user ID (uid) must be greater than or equal to zero to be valid. Only
strictly positive process IDs are valid. Add argument checks in
libprocessgroup of uid and pid arguments to make it easier to determine
the origin of invalid arguments.

Change-Id: I8a6d96ca4576bc9c329498c6a804dd05a02afca5
Signed-off-by: Bart Van Assche <bvanassche@google.com>