Merge with FreeBSD (RELENG_4) src/sys/dev/syscons/syscons.c,

Merge ... FreeBSD (RELENG_4) src/sys/dev/syscons/syscons.c,
revisions 1.336.2.16 and 1.336.2.17.

This fixes two bell-related bugs in syscons(4):

- Do not attempt to ring the audible bell if either the frequency
or duration is zero, as these values indicate that the bell is off,
yet can produce odd sounds on some hardware.  (1.336.2.16)

- Do not prevent processes on currently inactive VTY's from waking
up from waiting for the output queue when the visible bell rings on
the currently active VTY.  (1.336.2.17)

Reviewed-by: Hiten Pandya <hmp@backplane.com>