Support running with child namespaces

By default pgrep/pkill should not kill processes in a namespace it is not
part of. If this is allowed, it allows callers to break namespaces they did
not expect to affect, requiring rewrite of all callers to fix.

So by default, we should work in the current namespace. If --ns 0 is
specified, they we look at all namespaces, and if any other pid is specified
we continue to look in only that namespace.

Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>
2 jobs for namespace in 1 minute and 55 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #12788616
MakeCheck

00:01:55

failed #10976321
MakeCheck

00:02:39