path: add mp_find_config_file and reorganize some of the code

Add `mp_find_config_file` to search different known paths and use that in
ass_mp to look for the fontconfig configuration file.

Some incidental changes spawned by this feature where:

 * Buffer allocation for the strings containing the paths is now performed
   with talloc. All of the allocations are done on a NULL context, but it still
   improves readability of the code.
 * Move the OSX function for lookup inside of a bundle: this code path was
   currently not used by the bundle generated with `make osxbundle`. The plan
   is to use it again in a future commit to get a fontconfig config file.

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