Avoid unsigned integer overflow which can cause
rman_reserve_resource_bound() to return incorrect results.

Continue the initial search until the first viable region is found.

Add a comment to explain the search termination test.

PR:		kern/188534
Reviewed by:	jhb (previous version)
MFC after:	1 week
Virtual machine halt detection is turned on by default. Allow it to be
disabled via the tunable 'hw.vmm.halt_detection'.