netcat: Allow service names to be used.

Someone should really do a vendor import, but it's non-trivial, as we
have local modifications. In the meantime, here's a nine-year-old
upstream patch which allows service names to be used instead of port
numbers.

MFC after: 1 week
Obtained from: OpenBSD
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D50348

(cherry picked from commit 6d3d1fc3a30453be19831f79399bcba0ae822ad1)