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/os_unix.c b/src/os_unix.c
index 9d8d466..194f4be 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3780,7 +3780,11 @@
     }
 #endif
 
-    if (ttym_flags == TTYM_SGR)
+    if (T_CXM != NULL && *T_CXM != NUL)
+    {
+	term_enable_mouse(on);
+    }
+    else if (ttym_flags == TTYM_SGR)
     {
 	// SGR mode supports columns above 223
 	out_str_nf((char_u *)(on ? "\033[?1006h" : "\033[?1006l"));