vt(4): Connect to teken's TP_SETBELLPD

Add the glue needed to listen to TP_SETBELLPD which teken uses to
inform its client drivers about the results of parsing
\e[=<pitch>;<duration>B. It converts these to a Hz value for the
tone/pitch of the bell and a duration in ms. There's some loss of
precision because <pitch> in the escape seuquence is defined to be
(1193182 / pitch) Hz and <duration> is in 10ms units. Also note that
kbdcontrol also parses 'off' but then doesn't send the proper escape
sequence, leading me to wonder if that's another bug since teken
appears to parse that sequence properly and I've added code here to
treat that as the same as quiet or disabled.

In general, Hz from 100 to 2000 is good. Outside that range is possible,
but even at 100Hz the square wave is starting to sound bad and above
2000Hz the speaker may not respond.

Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D32620