nfs-utils: have mountd hold open etab file to force inode number to change

This patch changes mountd to hold the etab file open so that when it's
changed by exportfs, the inode number should change. We then change
auth_reload to reload the file based on whether st_ino is different
from the last time it was checked. It also changes auth_reload to
maintain a static counter value and return it instead of a timestamp
and fixes up get_exportlist accordingly. Finally, it adds some
comments to xtab_write to warn people about editing the etab in place.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>