Support of relative paths sourcing and cyclic source detection

Support of relative paths sourcing and cyclic source detection

Added HEAP/FILO structure and global singleton to contain the the list
of config files that have been sourced, and detect cyclic sourcing.
Implemented with a simple heap API:

- `path_heap_push()`
- `path_heap_pop()`
- `path_heap_front()`
- `path_heap_find()`

Added a new method `to_absolute_path()` to convert a path into an absolute
path, relatively to another config file's path. Using POSIX methods to
do so, adding libgen's header in `mutt.h`.

Closes #282
Closes #290