flua: lposix: add fnmatch function

The fnmatch function matches a string against a shell-style filename
pattern. It is a complex function and cannot easily be implenented
using regular expressions. Adding fnmatch to flua increases the amd64
binary by less than 1 KB.

Approved by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46849