ttymon should use tty-mode property
Update ttymon to identify serial device behind /dev/console and use ttyX-mode property value to set up the line.
This implementation is allowing ttymon to fetch line defaults from /etc/ttydefs, if we do have ttyX-mode property, merge it with line properties to use currently configured line speed.
Testing done: build/install/boot, confirm console setup and functionality.
For test, I did set up serial console redirection from BIOS (supermicro X10SAE, no BMC, however), and did remove console variable setup from /boot/solaris/bootenv.rc and loader config files. In both UEFI and BIOS boot, console variable was set to use serial port (tested both ttya and ttyb, physical ports on this hardware) and ttymon did pick up correct settings and console was usable on serial through poweron to console login (tested with default speed of 115200). With redirection disabled, the system defaulted to use local console (vga) again.
Dual console (serial + vga) enables access to console while boot loader is running - so we can use vga in case there is something wrong about serial.