do_mount_entry: add nosuid,nodev,rdonly flags if needed at remount

The kernel refuses mounts if we don't add those flags if
they were already there..

Also use the newly found info to skip remount if unneeded.

null_endofword() and get_field() were not changed, only moved up in
the file.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>