libkmod-file: refactor code to avoid ifdef mess.

libkmod-file: refactor code to avoid ifdef mess.

Refactor code to use pointer to functions, avoiding the previous

Now comp_types defines a magic header to be checked (size and bytes),
with the associated load() and unload() operations. If a header
matches, their operations are used. Otherwise the regular file
operations (mmap/munmap) are used.

File descriptor close is managed by the common code if it's valid
(>=0). If some code steals the file descriptor (eg: gzopen), then they
must change file->fd to -1.

This way the code should be easier to extend and avoid bugs.