python-linux-procfs: pflags: Ignore non-existent pids or process names

If the user enters a non-existent pid or process name, skip over it,

Also, if the user enters nothing but a non-existent pid, then make sure
the max_comm_len defaults to 0 instead of generating an error.

Signed-off-by: John Kacur <jkacur@redhat.com>