[PATCH] Bug 1961 pt2: store URI userinfo (aka login) in class URL

This continues the bug 1961 redesign of URL handling.

Move the HttpRequest::login detail into class URL as an SBuf. Renaming
to userInfo inline with the RFC defined name for this URI piece.

Converted the details to SBuf gaining several minor str*() removals in
the process and a simpler FTP login parser based on SBuf capabilities.

Also, updated the base64 encoder API used for converting between URI
userInfo and Basic authentication header token. Gaining better control
over the length of maximum token size and a small speedup from pre-known
input length.

Amos