Invoke the dirname() function in a POSIX compliant way.

POSIX requires that the argument of dirname() is of type "char *". In
other words, the input buffer can be modified by the function to store
the directory name.

Pull a copy of the string before calling dirname(). We don't care about
freeing up the memory afterwards, as this is done at the very bottom of
main(), right before the program terminates.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D6628