Turned out that as "extension" (which is really more like a pattern
that matches the whole file) was even used with patterns like "*.*"
so we do proper matching using fnmatch() now - which is even easier
than the old way.
Now deleting savegames on POSIX works.
In Sys_ListFiles() the extension was abused to match whole
filenames (of savegames), not just their extensions - that
didn't work with the POSIX backend, now it does.
Sys_ListFiles() now also supports "*" as a wildcard that matches
all files - needed for deleting savegames.
However, things like "bl*" or "*ub" don't work.
While at it, I replaced readdir() with the thread-safe readdir_r()
