Add new function ttyinitmode() which sets our systemwide default
modes on a tty structure. Both the ".init" and the current settings
are initialized allowing the function to be used both at attach and
open time.

The function takes an argument to decide if echoing should be enabled
by default. Echoing should not be enabled for regular physical
serial ports unless they are consoles, in which case they should
be configured by ttyconsolemode() instead.

Use the new function throughout.