Add assert that ce->ce_func is not NULL.

Match the docs: MUTEX_DRIVER/SPIN are now only for porting code written
for Solaris.

Merge some of the less invasive changes from the vmlocking branch:

- kthread, callout, devsw API changes
- select()/poll() improvements
- miscellaneous MT safety improvements

add a comment telling which structure members are zero-initialized,
as requested by YAMAMOTO Takashi

zero-init callback_head by memset(). code is 12 bytes shorter

callback_head_init: don't forget to initialize ch_running and ch_flags.
fix a problem reported by Jeff Rizzo on tech-kern@.

move wait points for kva from upper layers to vm_map. PR/33185 #1.

XXX there is a concern about interaction with kva fragmentation.
see: http://mail-index.NetBSD.org/tech-kern/2006/05/11/0000.html