Combined handling of fstab and mtab, improved caching

This patch reworks several aspects of the fstab and mtab handling.
1. Most importantly, it takes the union of fstab and mtab to create a
list of available shares.
2. Mtab is cached and keyed by share name, as fstab already is
3. FstabManager serves as a central dispatcher for mtab/fstabChanged
signals. Caches are invalidated, and recreated when needed.
4. Caches and valid flags are members of the FstabHandling class. Valid
flags are needed to differentiate between "empty and valid" and invalid
caches.

REVIEW: 103956
REVIEW: 103957
CCBUG: 290755