The /etc/mtab file may have mount information such as 'nfsd'
that if copied to /etc/fstab will cause the share server to
hang when rebooted.
Update /etc/fstab with exactly the newly mounted or unmounted
shares rather than simply overwriting /etc/fstab with /etc/mtab.
Change-Id: I67602bae1f928769d768008deca7bd0f2fef1ac2
Close-Bug: #1639662
(cherry picked from commit b76934770b)
(cherry picked from commit 344d4aa91c)
(cherry picked from commit 7b306c0676)
891d2deeee