• Refuse (overridable) to build ports on unsupported system version

Unfortunately, it's not uncommon for FreeBSD users to not update
their systems timely and thus end up using unsupported FreeBSD
release. These users continue to update ports tree as usual and
expect it to work, either unaware of the release EoL, or not clearly
understanding the consequences, which results in unexpected build
failures, bogus bug reports, attempts to bring back removed legacy
support bits and general discontent.

This change introduces system version check which makes ports refuse
to build anything on unsupported system. This makes users aware of
EoL of their system and makes it clear that no port is guaranteed to
build.

The error message tells how to override the check (by defining
ALLOW_UNSUPPORTED_SYSTEM, in which case it's turned into a simple
warning), additionally stressing that this configurartion is not
supported.

Currently outdated are OSVERSION < 1003000 (pre 10.3-RELEASE) and
1100000 <= OSVERSION < 1100122 (from 11-CURRENT'2013 to 11.0-PRERELEASE)

I expect these to be kept up to date with base system lifetimes,
be updated BEFORE removing any support for outdated release from
the tree and also serve as a reference of which OSVERSION checks
may be removed.

Approved by: portmgr (swills, mat)
Differential Revision: D9210