Bug 1961 part 2: redesign of URL handling.

Bug 1961 part 2: redesign of URL handling.

Move the HttpRequest::login detail into class URL. Renaming to userInfo
as per the RFC 3986 defined name for this URI piece.

Convert 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.