* amd/conf.c (gopt_forced_unmounts): check Linux kernel version

* amd/conf.c (gopt_forced_unmounts): check Linux kernel version
and alert if your version may be too old for MNT_FORCE to work
(before 2.4.0) or for MNT_DETACH to work (before 2.6.0).
Otherwise it may be impossible to pin down the exact kernel
version in which we should enable this feature.

* conf/umount/umount_linux.c (umount2_fs): if MNT_FORCE returned
EBUSY, then don't try to stat(2) before MNT_DETACH because it
could hang.