Add VSWTC

Only on Linux, Documented as VSWTCH (also added), as a SysV-only
thing but actually defined a VSWTC, must be disabled (ie: set to
_POSIX_DISABLE) to not show up in stty as being modified.

*sigh*

I'm not convinced Linux developers want termios to work.

src/syncterm/conn_pty.c

@@ -38,7 +38,11 @@
  #define CEOF CTRL('d')
 #endif
 #ifndef CEOL
+ #ifndef _POSIX_VDISABLE
   #define CEOL 0xff /* XXX avoid _POSIX_VDISABLE */
+ #else
+  #define CEOL _POSIX_VDISABLE
+ #endif
 #endif
 #ifndef CERASE
  #define CERASE 0177
@@ -103,6 +107,14 @@
  #define CFLUSH CDISCARD
 #endif
 
+#ifndef CSWTC
+ #ifndef _POSIX_VDISABLE
+  #define CSWTC 0xff /* XXX avoid _POSIX_VDISABLE */
+ #else
+  #define CSWTC _POSIX_VDISABLE
+ #endif
+#endif
+
 #ifndef TTYDEF_IFLAG
  #ifndef IMAXBEL
   #define TTYDEF_IFLAG (BRKINT | ICRNL | IXON | IXANY)
@@ -401,6 +413,12 @@ pty_connect(struct bbslist *bbs)
 #ifdef VMIN
 	ts.c_cc[VMIN] = CMIN;
 #endif
+#ifdef VSWTCH
+	ts.c_cc[VSWTCH] = CSWTCH;
+#endif
+#ifdef VSWTC
+	ts.c_cc[VSWTC] = CSWTC;
+#endif
 #ifdef VEOF
 	ts.c_cc[VEOF] = CEOF;
 #endif