selftests: mptcp: add port argument for pm_nl_ctl

This patch adds a new argument for pm_nl_ctl tool. We can use it like
this:

 # pm_nl_ctl add 10.0.2.1 flags signal port 10100
 # pm_nl_ctl dump
 id 1 flags signal 10.0.2.1 10100

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>