Stop using the non-standard basename_r() function.

This change makes the code use the POSIX basename() function. It has the
advantage that (if implemented correctly), it also imposes no restrict
on the pathname length.

Notice that I haven't added any error handling to the strdup() call. It
looks like none of the other calls to strdup() and malloc() performed by
this utility do it either.

Reviewed by: hrs
Differential Revision: https://reviews.freebsd.org/D6626