Fourth of several cleanups to soft dependency implementation.
Add KASSERTS that soft dependency functions only get called
for filesystems running with soft dependencies. Calling these
functions when soft updates are not compiled into the system
become panic's.

No functional change.

Tested by:    Peter Holm and Scott Long
Sponsored by: Netflix
The Xen userland event channel driver isn't complete.  Disabled it
from the kernel build until it is ready.

sys/conf/files:
	Remove the entry for xen/evtchn/evtchn_dev.c so it is not included
	in any kernel builds.

Noticed by:	smh
Avoid sign overflow if there are more than 2 GB of RAM.