rewrite guide: document URL encoding/decoding pipeline

rewrite guide: document URL encoding/decoding pipeline

tech.xml: new "URL Encoding and Decoding" section explaining that
httpd unescapes the URI before pattern matching, how to use
%{THE_REQUEST} for the raw form, AllowEncodedSlashes options, and
a summary of the [B]/[BNP]/[NE] flags with links to flags.xml.

flags.xml: add cross-references to tech.html#encoding from the [B],
[BNP], and [NE] flag sections. Restore section headers for flag_bnp
and flag_bctls that were inadvertently dropped.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934202 13f79535-47bb-0310-9956-ffa450edef68