ifdef shuffling: TIOCMC[GS]ET ioctls are not specified by POSIX,
so we can't assume they are present just because we HAVE_TERMIOS_H.
Conversely, if they are present then the subsidiary flags
TIOCM_{DTR|RTS} are almost certainly there as well, so that
ifdeffery can be removed.

And lastly, ifdefs for TIOCSBRK/TIOCCBRK are still needed.
(Those are logically separate functions even though TIP#35
lumped them together with DTR and RTS in -ttycontrol.
POSIX provides tcsendbreak() for this purpose, but that
interface doesn't fit with the TIP#35 API.)

KNOWN DEFECT: if a hypothetical Unix system is missing TIOCMCGET
but has TIOCSBRK/TIOCCBRK, the latter function will nevertheless
be unavailable.  Accounting for this possibility does not strike
me as being worth the ifdefs.