patch 9.0.0980: the keyboard state response may end up in a shell command

Problem:    The keyboard state response may end up in a shell command.
Solution:   Only request the keyboard protocol state when the typeahead is
            empty, no more commands are following and not exiting.  Add the
            t_RK termcap entry for this.
diff --git a/src/termdefs.h b/src/termdefs.h
index 133641b..9e9601f 100644
--- a/src/termdefs.h
+++ b/src/termdefs.h
@@ -69,6 +69,7 @@
     KS_KE,	// out of "keypad transmit" mode
     KS_TI,	// put terminal in termcap mode
     KS_CTI,	// put terminal in "raw" mode
+    KS_CRK,	// request keyboard protocol state
     KS_TE,	// end of termcap mode
     KS_CTE,	// end of "raw" mode
     KS_BC,	// backspace character (cursor left)
@@ -177,6 +178,7 @@
 #define T_KE	(TERM_STR(KS_KE))	// out of "keypad transmit" mode
 #define T_TI	(TERM_STR(KS_TI))	// put terminal in termcap mode
 #define T_CTI	(TERM_STR(KS_CTI))	// put terminal in "raw" mode
+#define T_CRK	(TERM_STR(KS_CRK))	// request keyboard protocol status
 #define T_TE	(TERM_STR(KS_TE))	// end of termcap mode
 #define T_CTE	(TERM_STR(KS_CTE))	// end of "raw" mode
 #define T_BC	(TERM_STR(KS_BC))	// backspace character