Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

Drop 'Set to' from most src.conf(5) knobs

The description is clearly what effect the knob has when set, so the
additional text was unnecessary.

Reviewed by: jhb, se
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29583

Adjust code based on comments from @asomers in D19551

Add MK_GOOGLETEST option for building/installing gmock/gtest

These libraries don't compile on non-C++-11 capable compilers, e.g., g++ 4.2.1
and its corresponding implementation of the c++ library, i.e., libstdc++.

Blacklist compilation on all non-C++-11 capable compilers and give others the
option of opting out of building/installing gmock/gtest via MK_GOOGLETEST.

This option is controlled by MK_CXX and MK_TESTS, as ATF compilation is.