afs: clarify cold and warm shutdown logic

afs: clarify cold and warm shutdown logic

Currently, any code that wants to perform a cold shutdown must first set
global afs_cold_shutdown = 1, then call afs_shutdown(void).

Instead, modify afs_shutdown() to accept a single parm which specifies
AFS_WARM or AFS_COLD shutdown, and to set the value of global
afs_cold_shutdown based on this parm.  Remove all other assignments for
afs_cold_shutdown.  Modify all callers of afs_shutdown() to specify
AFS_WARM or AFS_COLD as needed to maintain equivalent function.

This should make it much easier to tell at a glance what type of
shutdown is being requested by each caller to afs_shutdown().

No functional change should be incurred by this commit.

Change-Id: I921eca5b4d2659209154fbe37d575db69bf708b8
Reviewed-on: https://gerrit.openafs.org/12182
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>