9term: Set TERM_PROGRAM to termprog

TERM_PROGRAM is the customary way to identify which kind of terminal
emulator program one uses on macOS.
This change sets TERM_PROGRAM to termprog since both variables are used
for the same purpose.

src/cmd/9term/rcstart.c

@@ -87,6 +87,7 @@ rcstart(int argc, char **argv, int *pfd, int *tfd)
 	// Set $termprog to 9term or win for those who care about what kind of
 	// dumb terminal this is.
 	putenv("termprog", (char*)termprog);
+	putenv("TERM_PROGRAM", (char*)termprog);
 
 	pid = fork();
 	switch(pid){