Re-implement virtual console (constty).

Protect conscallout with tty lock instead of Giant. In addition to
Giant removal it also closes race on console unset.

Introduce additional lock to protect against concurrent console sets.

Remove consbuf free on console unset as unsafe, making impossible to
change buffer size after first allocation. Instead increase default
buffer size from 8KB to 64KB and processing rate from 5Hz to 10-15Hz
to make the output more smooth.

MFC after: 1 month