mount: create loop devices with LO_FLAGS_AUTOCLEAR flag

mount: create loop devices with LO_FLAGS_AUTOCLEAR flag

The "autolooped" mount (mount [-oloop] IMAGE /DIR/DIR)
always creates AUTOCLEARed loopdevs, so that umounting
drops them (and this does not require any code in the
umount userspace).
This happens since circa linux-2.6.25:
	commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
	Date:    Wed Feb 6 01:36:27 2008 -0800
	Subject: Allow auto-destruction of loop devices
IOW: in this case, umount does not have to use -d
to drop the loopdev.

The explicit loop mount (mount /dev/loopN /DIR/DIR)
does not do this. In this case, umount without -d
should not drop loopdev.
Unfortunately, bbox umount currently always implies -d,
this probably needs fixing.

function                                             old     new   delta
set_loop                                             537     597     +60
singlemount                                         1101    1138     +37
losetup_main                                         419     432     +13
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 110/0)             Total: 110 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>