fnmatch: use the system one if available

If configure detects fnmatch to be available, use that instead of our
custom one for FTP wildcard pattern matching. For standard compliance,
to reduce our footprint and to use already well tested and well
exercised code.

A POSIX fnmatch behaves slightly different than the internal function
for a few test patterns currently and the macOS one yet slightly
different. Test case 1307 is adjusted for these differences.

Closes #2626