libhammer - Preparation for snapshot handling & fixes

libhammer - Preparation for snapshot handling & fixes

Trivial changes:
	- Function renaming to actually match what they are
	  describing.
	- Clearer variable naming.

Additions:
	- Structures to hold snapshot data.
	- Functions to retrieve snapshots from metadata for PFSes.
	- A file for version compatibility functions.
	- A function to return the canonical access path of a
	  PFS which is suitable for opening.
	- Wrappers to operate per PFS snapshot list.

Changes & fixes:
	- Use the new functions to gather all snaps of a PFS.
	- Adapt hammer program for the changes.
	- Check before free'ing in libhammer_free_fsinfo()
	- Display the number of snapshots for unmounted PFSes.

Still missing:
	- Manpages changes.