js.exec() will now execute scripts from the configured mods and exec dirs

... if not passed a full path and the script file doesn't exist in the
startup_dir (if specified) or the same directory the calling script was
executed from (js.exec_dir).

This should fix the reported issue when trying to use the "spy" command from
a cmdshell script run from the mods dir:

!JavaScript  /sbbs/exec/str_cmds.js line 388: Error: Script file (mqtt_spy.js) does not exist

src/sbbs3/js_internal.c

@@ -416,8 +416,18 @@ js_execfile(JSContext *cx, uintN argc, jsval *arglist)
 					path[0] = 0;
 				JS_RESUMEREQUEST(cx, rc);
 			}
-			if (*path == '\0')
-				SAFECOPY(path, cmd);
+			if (*path == '\0') {
+				if (gptp->cfg->mods_dir[0] != '\0') {
+					snprintf(path, sizeof path, "%s%s", gptp->cfg->mods_dir, cmd);
+					if (!fexistcase(path))
+						*path = '\0';
+				}
+				if (*path == '\0') {
+					snprintf(path, sizeof path, "%s%s", gptp->cfg->exec_dir, cmd);
+					if (!fexistcase(path))
+						SAFECOPY(path, cmd);
+				}
+			}
 		}
 	}
 	free(cmd);