usock: add usock_port() for convenient use of numeric ports

usock: add usock_port() for convenient use of numeric ports

Add a new helper function usock_port() which converts the given numeric port number
into a string using a private static buffer. This way a calling application can
conveniently use numeric port arguments without having to convert them before:

  int fd = usock(USOCK_UDP, "example.org", usock_port(80));

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>