gfapi: support for chdir() and family

Add support for chdir, fchdir, getcwd, realpath equivalents in
GFAPI. These are required for the Samba VFS plugin.

Change-Id: I91d2db9146994403c98961c489c7640c51d5add2
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4853
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>