mc: use $termprog to identify 9term

TBR=rsc
https://codereview.appspot.com/112890043

src/cmd/draw/mc.c

diff --git a/src/cmd/draw/mc.c b/src/cmd/draw/mc.c
index de0f9bc..3f9c21f 100644
--- a/src/cmd/draw/mc.c
+++ b/src/cmd/draw/mc.c
@@ -297,7 +297,7 @@
 		return;
 	}
 
-	if((p = getenv("TERM")) != nil && strcmp(p, "9term") == 0)
+	if((p = getenv("termprog")) != nil && strcmp(p, "9term") == 0)
 	if((p = getenv("font")) != nil)
 		font = openfont(nil, p);