Add a separate "area tag" string configured per sub-board

According to  Lupine Furmen (FURFOL):
It says in the help that the field is used for both the Newsgroup name AND the
Fido Echo-tag.

These really should be 2 different fields.

Now, the echo/area tag is usually configured in the area file (areas.bbs) and so does not need to match *any* string configured in SCFG. But since an area file *can* be generated from SCFG (Export Areas option) and its possible to use SBBSecho withOUT an area file at all, then it's possible someone would like to have their FidoNet-style area tags configured different from any other strings related to a sub-board (e.g. newsgroup name) - why? I'm not clear, but there it is: another string that  a sysop can set (or not).

Since I'd recently added an area tag field per directory for use by hatchit.js, this does create some logical symmetry. And there were enough free bytes per-sub in the msgs.cnf, so why not.