cleanup UUID types V2

[Christoph Hellwig]

Hi all,

this series, which is a combined effort from Amir, Andy and me introduces
new uuid_t and guid_t type names that are less confusing than the existing
types, adds new helpers for them and starts switching the fs code over to
it.  Andy has additional patches on top to convert many of the users
that use char arrays for UUIDs and GUIDs to these (or rather a predecessor
for now until updated).

Changes since V2:
 - various cleanups