agetty: split out speed table and baud code to tty.c

agetty: split out speed table and baud code to tty.c

Move speedtab[], struct Speedtab, bcode() and list_speeds() to a
new tty.c file as agetty_bcode() and agetty_list_speeds(). The
speed table is kept static in tty.c.

Add agetty_fprint_speed() to print the numeric baud rate for a
given speed code, replacing the open-coded speedtab[] lookup in
output_special_char() case 'b'.

Signed-off-by: Karel Zak <kzak@redhat.com>