Improve compatibility of calls to dirname() on constant strings.

As the xinstall(8) utility had to be patched up to work with the POSIXly
correct basename()/dirname() prototypes, we make it pretty hard to build
previous versions of FreeBSD on HEAD. xinstall(8) is part of the
bootstrap tools.

Add some logic to <libgen.h> to automatically detect bad calls to
dirname() based on the type of the argument. If the argument is of type
'const char *', we simply fall back to calling into dirname@FBSD_1.0
directly.

I'll also give basename() similar treatment when importing the
thread-safe version of that function.

Tested by: bdrewery, madpilot (thanks!)