Change the management of commands in the active commands TAILQ to let issuers

Change the management of commands in the active commands TAILQ to let issuers
of synchronous commands perform the TAILQ_REMOVE of the command themselves,
instead of relying upon this being done for us if tsleep() returns zero.

Since we momentarily set `cold' again around suspend, tsleep() becomes a
no-op, which broke this assumption, and in turn caused TAILQ corruption,
with items being put on the freelist while still on the active list.

Found the hard way by ray@ playing with wsmoused after resume.