Refine the dirname(3) compatibility workaround a bit more.

Right now our workaround is so good that it doesn't throw any warnings
on misuse. This means that people will keep on using the old version
of dirname(3) silently without fixing their code.

Go ahead and change the prototype of __old_dirname() to also use a plain
char *, so that we still get a compiler warning. This won't have any
negative effect on building older versions of FreeBSD on HEAD, as those
are built with -Werror disabled.

Differential Revision: https://reviews.freebsd.org/D7844