Add support for symbol table in .com files

This change fixes minor bugs and adds a feature, which lets us store the
ELF symbol table, inside the ZIP directory. We use the path /zip/.symtab
which can be safely removed using a zip editing tool, to make the binary
smaller after compilation. This supplements the existing method of using
a separate .com.dbg file, which is still supported. The intent is people
don't always know that it's a good idea to download the debug file. It's
not great having someone's first experience be a crash report, that only
has numbers rather than symbols. This will help fix that!

tool/build/zipobj.c

@@ -52,7 +52,7 @@ struct timespec timestamp;
 wontreturn void PrintUsage(int rc, FILE *f) {
   fprintf(f, "%s%s%s\n", "Usage: ", program_invocation_name,
           " [-n] [-B] [-C INT] [-P PREFIX] [-o FILE] [-s SYMBOL] [-y YOINK] "
-          "[FILE...]\n");
+          "[FILE...]");
   exit(rc);
 }