If a key in the [feeds] section of modopts.ini begins with '_crosspost',
the value is assumed to be: 'url,sub1,sub2...', for example:

[feeds]
_crosspost1 = http://my-feed/rss.xml,dove-gen,syncprog,general

Avoid duplicate key names by tacking a number onto the end:
_crosspost1, _crosspost2

The old style 'sub = http://my-feed/rss.xml' pairs are still supported.