I've added the variable CFG->main_frame to config-dist.php, and replaced _top
targets with the variable in all the php files.  HTML files, like those in the
documentation, have not been modified, as they are not (yet) parsed.

The variable should probably get moved into the config table, and get some
documentation.

I've also included, but commented out, a slight change in weblib.php, which
would guarantee that messages would be seen before redirecting the user (unless
delay intentionally set to 0 when redirect is used).