Rewrite timeout(9) to be callout(9)-centric instead. Move the description
of timeout(9) to the end and mark it prominently as deprecated. Document
somewhat how times are specified for the 'sbt' variants. Better explain
how using callout_init_*() to associate a lock with a callout resolves
common races.

Differential Revision: https://reviews.freebsd.org/D847
Reviewed by: wblock, bjk
MFC after: 1 week