util: function to get local nl_pid used by netlink event socket

util: function to get local nl_pid used by netlink event socket

This value will be needed to set the src_pid when sending netlink
messages to lldpad. It is part of the solution to:

  https://bugzilla.redhat.com/show_bug.cgi?id=816465

Note that libnl's port generation algorithm guarantees that the
nl_socket_get_local_port() will always be > 0 (since it is "getpid() +
(n << 22>" where n is always < 1024), so it is okay to cast the
uint32_t to int (thus allowing us to use -1 as an error sentinel).