Use basename() and dirname() right on FreeBSD. [skip appveyor]

Use basename() and dirname() right on FreeBSD. [skip appveyor]

On FreeBSD CMake runs Clang with -std=gnu99, which used to emit three
warnings in a Capsicum-specific block along the following lines:

tcpdump.c:2432:32: warning: '_Generic' is a C11 extension
  [-Wc11-extensions]
/usr/include/libgen.h:61:21: note: expanded from macro 'basename'
  basename)(x)
/usr/include/sys/cdefs.h:325:2: note: expanded from macro '__generic'
        _Generic(expr, t: yes, default: no)

(In the same context Autoconf does not request a specific C standard
from Clang, so it happens to use C17 and there is no warning.)

Type-generic expressions support in C99 mode seems to be a quirk of
Clang that cannot be disabled and is harmless in this context, so
introduce and use another pair of diagnostic control macros (for Clang
only) to squelch the warnings.

While at it, study the reason for _Generic use in /usr/include/libgen.h
and realize that tcpdump.c still assumes the behaviour before FreeBSD
12.0.  Add two temporary buffers around basename() and dirname() calls
to get consistent results regardless of which FreeBSD version it is.

Remove the last remaining exemption rule.