It could cause possible locking issue to call tty(4) layer functions
from IPL_SERIAL interrupt handlers.
Changes details (mostly similar to com(4) and z8530tty(4)):
- allocate RX buffer and put data and status into it in the interrupt handler
- call t_linesw->l_rint from the software interrupt handler
- set the only first byte in siottystart() and use t_outq buffer to send
multiple TX data
- call the next t_linesw->l_start on TX complete in the software interrupt
handler
Also put more changes:
- handle cnmagic(9) properly (now entering ddb(4) by console BREAK works)
- allocate tty in the attach function as other drivers rather than first open
- use proper variable types
bc63cd47b4