Convert test expression into less imposing form.
rejoin the branch
A bit more tidiness expressing the new test expression.
Somewhat tidier, less daunting test.
Somewhat tidier, less daunting test.
minor: formatting tweaks in the change log
termios cleanup step 1: remove USE_TERMIO and USE_SGTTY conditional sections
(mechanical change, done with `unifdef`).
For termios, we never want DIRECT_BAUD; always use the symbolic constants
as prescribed by POSIX.
ifdef reduction: SUPPORTS_TTY defined if and only if USE_TERMIOS defined.
ifdef reduction - missed a couple spots (#if ...  defined(USE_TERMIO))
Remove IOSTATE facade: it's always a struct termios.
TtyGetBaud(), TtyGetSpeed(): use POSIX speed_t typedef
instead of 'unsigned long'.
Allow Tcl to be compiled even if Tcl_Eval, Tcl_GetVar, ... are macros.
fix coroutine-4.6 so that it runs in isolation, [Bug 3606395]
Revise TclReleaseLiteral() to tolerate a NULL interp argument.
Update callers and revise mistaken comments.
TtyGetOptionProc: remove inoperative comment "The string returned by 
this function is in static storage [...]"; this is not the case
(and apparently never has been)
[Bug 3606542]: Add missing constraint to test.
[Bug 3606542]: Add missing constraint to test.
[Bug 3606397]: Make test work in isolation, and corrected what was being tested.
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.