patch 8.2.0856: CTRL-S stops output

Problem:    CTRL-S stops output.
Solution:   Invert the IXON flag. (closes #6166)
diff --git a/src/os_unix.c b/src/os_unix.c
index e1296c4..c6ba249 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3481,10 +3481,8 @@
     if (tmode == TMODE_RAW)
     {
 	// ~ICRNL enables typing ^V^M
-	tnew.c_iflag &= ~ICRNL;
-	// Do not make CTRL-S stop output, for most users it is unexpected and
-	// is hardly ever useful.
-	tnew.c_iflag |= IXON;
+	// ~IXON disables CTRL-S stopping output, so that it can be mapped.
+	tnew.c_iflag &= ~(ICRNL | IXON);
 	tnew.c_lflag &= ~(ICANON | ECHO | ISIG | ECHOE
 # if defined(IEXTEN) && !defined(__MINT__)
 		    | IEXTEN	    // IEXTEN enables typing ^V on SOLARIS