porting: Remove unnecessary code from mount_darwin.c

   Backport of http://review.gluster.org/8564

- Cleanup mount_darwin.c to make it cleaner
- Restructure the code to be more readable
- Avoid unnecessary delays invoking `mount_osxfusefs`

Change-Id: I7f28875b0ec872a08bf8e77dfc8ebe5eca750d0e
BUG: 1144163
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8772
Tested-by: Gluster Build System <jenkins@build.gluster.com>