Add safety checks on all hostList traversals

Add safety checks on all hostList traversals

Currently, h_Enumerate checks that it doesn't enumerate over more than
hostCount hosts, in case the hostList has a cycle or is otherwise
corrupt. Add similar checks to all places in the code that loop over
hostList, to prevent the code from getting in an infinite loop under
H_LOCK in the case of a hostList cycle.

Also, ShutDownAndCore instead of assert'ing, so we try and detach
volumes first, possibly reducing salvaging time when we restart after
core'ing.

(cherry picked from 58258ba3b89732ea825e1aff2decab54347ab92e)
Change-Id: Ide1e5aca7c2c4a4af3f62bc07821db694f2f9999
Reviewed-on: http://gerrit.openafs.org/863
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-on: http://gerrit.openafs.org/990
Reviewed-by: Andrew Deason <adeason@sinenomine.net>