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.