Add RELOAD sysop command to reload a (JS) command shell without logoff/on

We have to use load() (rather than js.exec) to invoke str_cmds.js so that an
exit() will actually exit. Since load() automatically does the mods vs exec
directory search-dance, that simplifies the code in default.js a little.
I'm not sure exactly why I originally chose to use js.exec() over load() for
invoking str_cmds.js, but for this feature, we need load() so let's go with
that for now.

I did encounter an issue (issue #840) while originally trying to make this
work with the original code that called js.exec(), but just punted and went
with load() instead. Perhaps if we fix issue #840, we can revert default.js
back to using js.exec() (but why we would need/want to, I'm not sure).

exec/default.js

@@ -203,10 +203,7 @@ while(bbs.online && !js.terminated) {
 		cmd = console.getstr();
 		if(cmd == '!')
 			cmd = last_str_cmd;
-		var script = system.mods_dir + "str_cmds.js";
-		if(!file_exists(script))
-			script = system.exec_dir + "str_cmds.js";
-		js.exec(script, {}, cmd);
+		load({}, "str_cmds.js", cmd);
 		last_str_cmd = cmd;
 		continue;
 	}

exec/str_cmds.js

@@ -351,6 +351,11 @@ function str_cmds(str)
 			return;
 		}
 
+		if(str=="HELP")
+			writeln("RELOAD\tReload the current shell (if JavaScript).");
+		if(str=="RELOAD")
+			exit(0);
+
 		if(str=="HELP")
 			writeln("SLOG\tExecutes the slog utility to display system statistics.");
 		if(str=="SLOG") {