Add 3wire and std as terminal types/classes. These are similar to
the existing terminal types/classes that have the baudrate suffix,
but differ in that no baudrate is set/defined.

The purpose of these new types/classes is to allow them to be used
for the serial console. Currently the uart(4) driver fixates the
baudrate and the CLOCAL flag, which means that it doesn't matter
whether you give it std.<baud> or 3wire.<baud> as the terminal type
to getty and what exactly <baud> is set to. It's being overridden
by uart(4). The goal is to change uart(4) not to override these
settings.
Change the terminal type/class for enabled serial lines to 3wire. This
allows us to change the uart(4) driver to not hardcode specific line
settings for the serial console.

A terminal type of 3wire makes sure the console still works when no DCD
signal is present, which preserves behviour. When it is known that the
terminal server (or DCE in general) provides DCD, a terminal type/class
of std can be used. This has the effect of being logged out when one
disconnects from the console -- improving security overall.

Likewise, when uart(4) does not fixate the baudrate, one can change
the terminal type/class to set a specific baudrate. An operator can use
this to change the console speed mid-flight, without needing a reboot.
Of course it helps in this respect if and when the firmware can be
configured from the OS.

The above mentioned capabilities depend on uart(4) being changed, which
is to happen next.
Remove all traces of support for ARM chips prior to the arm9 series.  We
never actually ran on these chips (other than using SA1 support in an
emulator to do the early porting to FreeBSD long long ago).  The clutter
and complexity of some of this code keeps getting in the way of other
maintenance, so it's time to go.