patch 9.0.1168: code to enable/disable mouse is not from terminfo/termcap

Problem:    Code to enable/disable mouse is not from terminfo/termcap.
Solution:   Request the "XM" entry and use it to set 'ttymouse' if possible.
diff --git a/src/termdefs.h b/src/termdefs.h
index a258dad..0239965 100644
--- a/src/termdefs.h
+++ b/src/termdefs.h
@@ -95,6 +95,7 @@
     KS_CGP,	// get window position
     KS_CWS,	// set window size in characters
     KS_CRV,	// request version string
+    KS_CXM,	// enable/disable mouse reporting
     KS_RFG,	// request foreground color
     KS_RBG,	// request background color
     KS_CSI,	// start insert mode (bar cursor)
@@ -205,6 +206,7 @@
 #define T_CEI	(TERM_STR(KS_CEI))	// end insert mode
 #define T_CSR	(TERM_STR(KS_CSR))	// start replace mode
 #define T_CRV	(TERM_STR(KS_CRV))	// request version string
+#define T_CXM	(TERM_STR(KS_CXM))	// enable/disable mouse reporting
 #define T_RFG	(TERM_STR(KS_RFG))	// request foreground RGB
 #define T_RBG	(TERM_STR(KS_RBG))	// request background RGB
 #define T_OP	(TERM_STR(KS_OP))	// original color pair