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>