Add chmod() wrappers to library

Add chmod() wrappers to library

Add functions for changing/adding/removing permissions of files.

mutt_file_chmod          - set permissions
mutt_file_chmod_add      - add permissions
mutt_file_chmod_add_stat - add permissions
mutt_file_chmod_rm       - remove permissions
mutt_file_chmod_rm_stat  - remove permissions

The *_stat functions accept an additional optional stat structure
parameter, which must point to the stat structure of the file.
Using the *_stat functions can save a call to stat() as the stat
structure does not need to be queried by the mutt_file_chmod_* function.