terminal-util: refactor vt_disallocate()

terminal-util: refactor vt_disallocate()

Numerous fixes:

1. use vtnr_from_tty() to parse out VT number from tty path
2. open tty for write only when we want to output just ansi sequences
3. open tty in asynchronous mode, and apply a timeout, just to be safe
4. propagate error from writing (most callers ignore it anyway, might as
   well pass it along correctly)