EvaluateFreeSpace() Rubyfication
handle Btrfs specifically in disk usage counting (bnc#896176)

- use "btrfs filesystem df" instead of "df" (more accurate)
- pass "growonly" flag to libzypp when any snapshot is found
changes

- 3.1.48
code review fixes
code review fixes
more Rubyfication
Merge pull request #85 from yast/better_btrfs_support

Btrfs support in space calculation (bnc#896176)