Prior to this, you could get a viewer crash whose ERROR: message said only:
convert_from_llsd<std::string>: Invalid string value
This gave no hint as to *which value* was wrong, or where to go fix it.
Ironically, each convert_from_llsd<> specialization already has the
control_name and LLSD value in hand; added these to each such LL_ERRS message.

Shut down thread if viewer closed while downloading; fix problem of download marker path failing to expand correctly because it was happening too early in start up.

just barely working udate downloading service; missing little nicities like error checking and sill stuff like that.

This reduces the number of symbols in the viewer binary by about 30%.
Also, fixed test errors revealed by inlining being enabled.