src: Use gnu++17 as the default C++ standard

Previously the compiler's default C++ standard was used unlike C where
bsd.sys.mk explicitly sets a default language version. Setting an
explicit default version will give a more uniform experience across
different compilers and compiler versions.

gnu++17 was chosen to match the default C standard. It is well
supported by a wide range of clang (5+) and GCC (9+) versions.

gnu++17 is also the default C++ standard in recent versions of clang
(16+) and GCC (11+). As a result, many of the explicit CXXSTD
settings in Makefiles had the effect of lowering the C++ standard
instead of raising it as was originally intended and are removed.

Note that the remaining explicit CXXSTD settings for atf and liblutok
explicitly lower the standard to C++11 due to use of the deprecated
auto_ptr<> template which is removed in later versions.

Reviewed by: imp, asomers, dim, emaste
Differential Revision: https://reviews.freebsd.org/D49223