Use POSIXly correct "grep -E"

I would like to rant here about how it is not solving any problem to
remove "egrep" and the trend of making frequently used UNIX commands
harder to type is a serious problem, but that sounds kind of like
something a crazy person might say after a harrangue about their
diminished quality of life without Ma Bell's rotary phones, so I shall
desist and keep some outward semblance of sanity.

vv

@@ -540,7 +540,7 @@ inittermkey() {
 
     # To be valid, we must enable terminal application mode key sequences.
     if tput smkx; then
-	for k in $(infocmp -L1 | egrep 'key_|cursor_'); do
+	for k in $(infocmp -L1 | grep -E 'key_|cursor_'); do
 	    a=""; x=""
 	    # Example "k:	key_down=\EOB,"
 	    a=${k#*key_}