kbd: consolidate kb interfaces (phase one)

Refactor to eliminate duplicated rate and delay tables, with minor style
tweaks for changed lines. Remove an obsolete comment about needing to
convert from microseconds to ticks (that's done elsewhere). Remove
traiing whitespace in kbdcontrol.c.

Except for the new warning, no change in behavior

Sponsored by: DSS GmbH
Reviewed by: imp [minor style tweaks as well]
Pull Request: https://github.com/freebsd/pull/683
Differential Revision: https://reviews.freebsd.org/D38818