mi_switch(): clean up switch types and their usage

mi_switch(): clean up switch types and their usage

Overall, this is a non-functional change, except for kernels built with
SCHED_STATS. However, the switch types are useful for communicating the
intent of the caller.

1. Ensure that every caller provides a type. In most cases, we upgrade
   the basic yield to sched_relinquish() aka SWT_RELINQUISH.
2. The case of sched_bind() is distinct, so add a new switch type SWT_BIND.
3. Remove the two unused types, SWT_PREEMPT and SWT_SLEEPQTIMO.
4. Remove SWT_NONE altogether and assert that callers always provide
   a type flag.
5. Reference the mi_switch(9) man page in the comments, as these flags
   will be documented there.

Reviewed by:	kib, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D38184