mount: relative atime support

mount: relative atime support

Add the "relatime" (relative atime) option support to mount.  Relative
atime only updates the atime if the previous atime is older than the
mtime or ctime.  Like noatime, but useful for applications like mutt
that need to know when a file has been read since it was last
modified.

Cc: Adrian Bunk <bunk@stusta.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Karel Zak <kzak@redhat.com>
Signed-off-by: Valerie Henson <val_henson@linux.intel.com>