Completely reworked the path convertion functions

Moved everything (from sbr/getfolder.c and sbr/m_maildir.c) into
sbr/path.c, but actually replaced the code almost completely.
See h/prototypes.h for the function changes.
sbr/path.c provides explaining comments on the functions.
None of them allocates memory automatically.

Additionally:
- Like for other ``files'', `inc -audit file' places file relative
  to the cwd, not relative to the mh-dir. This is for consistency.
- Replaced add(foo, NULL) with getcpy(foo), which ist clearer.