Fix grep pattern when searching for listening sockets

The current grep pattern matches also port numbers that match only
partially the given $PORT number; e.g., if $PORT is 6080, 60800 will
match as well.

While TCP listening sockets in the 60000-65535 range are rare, they
need to be handled as well. The problem is also present if the user
selects a shorter PORT value with the --listen command line argument.

By adding a space, the pattern is fixed.