rename scheduler->swapper and SI_SUB_RUN_SCHEDULER->SI_SUB_LAST

Also directly call swapper() at the end of mi_startup instead of
relying on swapper being the last thing in sysinits order.

Rationale:

- "RUN_SCHEDULER" was misleading, scheduling already takes place at that stage
- "scheduler" was misleading, the function swaps in the swapped out processes
- another SYSINIT(SI_SUB_RUN_SCHEDULER, SI_ORDER_ANY) could never be
  invoked depending on its relative order with scheduler; this was not obvious
  and the bug actually used to exist

Reviewed by:	kib (ealier version)
MFC after:	14 days
zfsboottest: remove explicit -Werror

Unfortunately sys/boot is compliled without -Werror at the moment.

MFC after:	7 days
zfs module: perform cleanup during shutdown in addition to module unload

- move init and fini code into separate functions (like it is done upstream)
- invoke fini code via shutdown_post_sync event hook

This should make zfs close its underlying devices during shutdown,
which may be important for their drivers.

MFC after:	20 days