Damn HFS+, part I: use setuid bit instead of setgid
Move from modified-base64 encoded paths -> hexadecimal ASCII representation

The switch to hex+ASCII removes a testing annoyance on OS X:
the HFS+ file system is usually configured to be case preserving
but not case sensitive.  The use of both upper- & lowercase letters
by the modified-base64 (base64fs2.erl) can cause false conflicts.

I put the hexlist_to_binary/1 and binary_to_hexlist/2 functions
in the riak_kv_fs2_backend.erl module (where they don't really belong)
to avoid a module dependency when running EUnit & QuickCheck tests
for riak_kv_fs2_backend, which need to be run from the riak_kv source
repo.  There's still a dependency on include/riak_cs_lfs.hrl, which
needs fixing, but at least it's only one dep instead of two.
Add bucket prefix length, TYPICAL_BUCKET_PREFIX_LEN, et al.