openpty: provide a stub on mingw

openpty: provide a stub on mingw

On mingw, the compiler complained that 'struct termios' and
'struct winsize' were declared in the function prototype, then
failed to compile due to missing TCSAFLUSH.  Since we can't
emulate ptys on mingw, it's better to just make this module
be a stub that compiles but gracefully fails.

This patch assumes that the only portable way to use openpty()
is with the fourth and fifth arguments being NULL ('struct termios'
cannot be portably initialized in a standard-compliant manner
except by open(O_TTY_INIT)/tcgetattr(), and 'struct winsize' is
not standardized); for now, applications that want to alter termios
settings still have the burden of conditional compilation to avoid
the missing tcgetattr() on mingw.

* lib/pty.in.h (includes): Provide forward declarations.
* lib/openpty.c (openpty) [mingw]: Provide ENOSYS stub.

Signed-off-by: Eric Blake <eblake@redhat.com>