libglusterfs *at functions don't support AT_FDCWD semantics

[slowfranklin]

Description of problem:

All libglusterfs *at functions don't seem to support AT_FDCWD semantics. See man openat() for a description of these.

Is there a reason for this omission? The lack of support for AT_FDCWD semantics puts the burden of switching between *at and non-at function on the API user, which is far from ideal.