New bc and dc implementation merged to 12-STABLE (off by default)

[Stefan Esser]

The new bc and dc implementation that has become the default version in
FreeBSD-CURRENT has been merged to 12-STABLE and will be included in the
upcoming release 12.2.

It has a number of advantages (standards compliance, speed, NLS support)
over the one in previous FreeBSD releases, but in accordance with POLA
it will require the option WITH_GH_BC=yes to be passed to buildworld and
installworld in FreeBSD-12.

With WITH_TESTS=yes a large number of tests is built and available as
part of the Kyua based test framework - test results from platforms
other than i386 and amd64 are welcome.

The same version of bc and dc is also available as a port or package,
but built with less aggressive compiler optimizations and without
any test cases being installed (but they can be run with "make test"
in the port directory /usr/ports/math/gh-bc).