Add a safe_unmount function and use it in enter_chroot.

Add a safe_unmount function and use it in enter_chroot.

The safe_unmount function first tries a regular unmount,  If that
fails it warns and tries a lazy unmount.  If the lazy unmount fails
it dies.

Both unmounts take the -d option in case the mount is a loop device.

BUG=None
TEST=Entered and exited multiple chroots

[adlr, 2010-04-12 20:46:20]

http://codereview.chromium.org/1593021/diff/1/2
File src/scripts/common.sh (right):

http://codereview.chromium.org/1593021/diff/1/2#newcode348
src/scripts/common.sh:348: sudo umount -d "${path}"
how does this play w/ "set -e" in parent scripts?

[robotboy, 2010-04-12 20:59:10]

I think the "set -e" doesn't make it to sourced scripts because my tests worked.
 But I've changed it for consistency sake.

http://codereview.chromium.org/1593021/diff/1/2
File src/scripts/common.sh (right):

http://codereview.chromium.org/1593021/diff/1/2#newcode348
src/scripts/common.sh:348: sudo umount -d "${path}"
In my tests it worked correctly.  That is when the umount failed I saw the
warnings and the lazy unmount happened.  But for consistency sake I've changed
it.

Thanks.

[adlr, 2010-04-12 21:02:00]

LGTM