Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFV: nc(1) from OpenBSD 4.8.

While I'm there, bump WARNS level to 2 as the vendor
have the right printf format string now.

MFC after: 1 month
Obtained from: OpenBSD

Build usr.bin/ with WARNS=6 by default.

Also add some missing $FreeBSD$ to keep svn happy.

Fix bugs in the previous commit.

Revamp several Makefile nits:
- .PATH: is spelled ``.PATH: ''
- Don't forget to use DPADD so ``make checkdpadd'' is
not broken.
- LDADD should not come with + as it's empty in the
first place

Suggested by: ru

Finish the import of nc(1) from OpenBSD. This includes:
- Bring IPsec support from the ports collection [1].
- Bring -o ("once only") option from the ports
collection [2].
- Adopt the Makefile framework into
usr.bin/nc/Makefile.
- Add a knob to control whether to build nc(1),
NO_NETCAT.
- Bump __FreeBSD_version so ports collection can
detect this change.

Original patchset are contributed to the ports collection by:
[1] nectar, [2] joerg.

Note: WARNS?=6 patchset spined off in this commit, in order not
to take too many files off the vendor branch.