old-school tty garbage collection: comcontrol

OK.  I've done the following three reviews

https://reviews.freebsd.org/D52315
https://reviews.freebsd.org/D52316
https://reviews.freebsd.org/D52317

which remove stuff in the tree that's no longer relevant: references to
some old drivers and the dtrwait attribute / ioctl. It's been gone for 15
years since the tty mpsafe rewrite.

But it only leaves 'drainwait' in comcontrol(8), and that's not been
relevant to modems since the late 90s (the ones that were trouble largely
disappeared from service by 2005). I wonder if we might not want to just
retire comcontrol entirely. This one feature is the sole-surviving remnant
from the 386BSD patchkit driver that Christopher G. Demetriou wrote that
FreeBSD 1 imported (and we reimported into FreeBSD 2).

Does anybody still use it? I think not.

And also speaking of things nobody uses: rc.d/serial has support for serial
mice (which I think we can remove)... in fact, it's one of the 'old-school
modify me' files that snuck into rc.d from prior rc systems. It's done
nothing by default since forever. And only the netif rc script references
it. These days, ppp or whatever should be doing the serial port frobbing
for serial connections. And even then, it seems like we don't need it. I'm
thinking of just removing it entirely. Does anybody still use this? You'll
be soon out-of-luck since pkgbase, I think, will just overwrite it.

Comments?

Warner