pty.c: Restore build on systems without cfmakeraw(3) (re: 5a2e7dae)

The OpenSUSE patch uses cfmakeraw(3) which is on Linux, BSD and
macOS, but not portable. The build failed on Solaris and variants.

src/cmd/builtin/features/pty:
- Add simple test for the presence of cfmakeraw(3). I love iffe.

src/cmd/builtin/pty.c:
- Add POSIX compliant fallback flaggery for systems without it.

src/cmd/builtin/features/pty

@@ -11,6 +11,7 @@ header	stropts.h
 
 lib	openpty,_getpty,ptsname -lutil
 lib	grantpt,unlockpt,posix_openpt stdlib.h
+lib	cfmakeraw termios.h
 
 tst - output{
 	#include	<sys/types.h>