Simplify the startup of the cleaner, reaper and update threads by

Simplify the startup of the cleaner, reaper and update threads by
passing the main function directly to kthread_create(9). The start_*
functions are mere stepping stones nowadays and can be pruned.
They used to contain more logic in the pre-kthread era.

While here, set `cleanerproc' and `syncerproc' during the thread
creation rather than expect the threads to set the proc pointer.
Also, rename `sched_sync' to `syncer_thread' to reduce confusion
with the scheduler-related functions.

OK kettenis@, deraadt@, mpi@