patch 8.0.0539: startup test fails on Mac

Problem:    Startup test fails on Mac.
Solution:   Use another term name, "unknown" is known. Avoid a 2 second delay.
diff --git a/src/proto/main.pro b/src/proto/main.pro
index f8abb94..278d1ad 100644
--- a/src/proto/main.pro
+++ b/src/proto/main.pro
@@ -1,6 +1,7 @@
 /* main.c */
 int vim_main2(void);
 void common_init(mparm_T *paramp);
+int is_not_a_term(void);
 void main_loop(int cmdwin, int noexmode);
 void getout_preserve_modified(int exitval);
 void getout(int exitval);