Refactor and cleanup sio (uPD7201) drivers.

- remove confusing syscn*() functions (and its header) and prepare
explicit siottycninit(), siottycnget() and siottycnput() functions
- use exported struct consdev and cn_tab to initialize siotty console
- enable and handle E/S interrupts to make BREAK signal detected properly
- make CSR (status regsiters) access functions inline static
- make single byte read/write (i.e. cnputc() and cngetc()) functions
static inline and take struct sio_register rather than sio channel
- use proper integer type (uint16_t) for getsiocsr() as siotty.c
- handle channel dependent CR2A and CR2B registers properly
- use more explicit definitions for RR_* macro used by getsiocsr()
- define and use proper RR0 (read register) values
(there is no isStatusReg(r) macro used on 4.4BSD/luna68k)

Tested on LUNA with both wscons console and serial console.