Remove unportable calls to basename().

The POSIX version of basename() doesn't use a 'const char *' argument;
the function may overwrite its input buffer. Instead of copying the
input string, let's just simplify this code by using our getprogname()
function that already returns the name of the application in the right
format.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D6094