Updated for webv4 and fix off-by-one error reported by LaF0rge

Make the scheme ("http://" or "https://") configurable via rss.ini, default is
now "https://".

Make the message link format somewhat configurable via rss.ini (hopefully to
support some other/future web message reading interface should there be one).

Reportedly, the MsgBase.get_msg_header/body() calls of the "total_msgs" offset
were logging errors? I was not able to reproduce this (the !hdr check seemed
to be successfully ignoring such cases), but in any case, message offsets are
0-based, so this was definitely an off-by-one issue, even if it was a silent
failure for everyone else. <shrug>