hacklib.c NONNULL functions

A bunch of routines return a pointer which is never Null but weren't
telling the compiler that such was the case.  A couple (strsubst(),
stripchars()) were accepting Null output argument and then returning
Null, but callers had no reason to use them that way, so they've been
changed.  (upstart() could have been changed similarly; I've already
forgotten why I left it as-is.)

src/files.c

@@ -3460,7 +3460,7 @@ config_erradd(const char *buf)
 
     /* if buf[] doesn't end in a period, exclamation point, or question mark,
        we'll include a period (in the message, not appended to buf[]) */
-    punct = eos((char *) buf) - 1; /* eos(buf)-1 is valid; cast away const */
+    punct = c_eos((char *) buf) - 1; /* eos(buf)-1 is valid */
     punct = strchr(".!?", *punct) ? "" : ".";
 
     if (!gp.program_state.config_error_ready) {