HTTP/1.1: unfold mime header blocks

HTTP/1.1: unfold mime header blocks

Enact the RFC 7230 section 3 requirement that proxies remove obs-fold from
received mime header blocks and drop all lines that start with whitespace
immediately following the request-line.

Also;

* Shuffle the DelimiterCharacters() and RelaxedDelimiters() helper methods
  to Http::One::Parser for use in processing mime.

* Extend the headersEnd() function with a wrapper to avoid SBuf::c_str()
  and to efficiently report whether obs-fold exists without needing to
  re-scan the block.

* Document the mime_header.h API function(s).

* Add Http::One::CrLf() to present a SBuf CRLF constant. This can be used
  instead of the many local string definitions we have now.
  The implementation could perhapse be done better for performance, but not
  done here.