unifdef.1: Use Mt.
kernel/spinlock: Add a description to struct spinlock.

And add it to spin_init() and SPINLOCK_INITIALIZER().

Submitted-by: dclink (see <http://bugs.dragonflybsd.org/issues/2714>)
OK'd-by:      dillon
test: Add bunch of tests for udp protocol

They test various combination of bind(2), connect(2) and sendto(2)
for both unicast and multicast.  Unicast tests work w/ udpecho in
tools/netrate.  Multicast tests work w/ mcastsend.