gfapi: add new API glfs_dup() to copy a file descriptor

Duplicate a glfs_fd file descriptor. Inherit the internal
fd_t with a reference.

Change-Id: Ib30e9a46b608b9f78202957f4dab6ba6265a9ec0
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5109
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>