Add support for command-line specifiers in external program 'startup dir'

The exception is: DOS programs on Linux (e.g. Linux DOSEMU), due to the
funky drive letter mappng, this can't be supported for those external
programs.

As requested by Fluid via IRC, for Frotz support, to store user-unique
data files in data/user/####.something (e.g. "%juser/%4.zork").

src/sbbs3/xtrn_sec.cpp

@@ -1425,7 +1425,7 @@ bool sbbs_t::exec_xtrn(uint xtrnnum, bool user_event)
 	else
 #endif
 	{
-		SAFECOPY(startup_dir, cfg.xtrn[xtrnnum]->path);
+		cmdstr(cfg.xtrn[xtrnnum]->path, "", "", startup_dir);
 		SAFECOPY(drop_file, path);
 	}