Update docs

This commit adds wildcard path matching.  If path pattern given in
backend option ends with "*", it is considered as wildcard path.  "*"
must match at least one character.  All paths which include wildcard
path without last "*" as prefix, and are strictly longer than wildcard
path without last "*" are matched.

Previously, the incoming invalid regular header field was ignored by
default.  With this commit, they are now treated as stream error, and
the stream is reset by default.  The error code used is now
PROTOCOL_ERROR, instead of INTERNAL_ERROR.

With this commit, SSL_TLSEXT_ERR_NOACK is returned from
servername_callback, which removes server_name extension from
ServerHello.  CertLookupTree is now used even if the number of server
certificate is one.  It is better to exercise it regularly.

nghttpx: Forward multiple via, xff, and xfp header fields

Previously, for Via, X-Forwarded-For, and X-Forwarded-Proto header
field, nghttpx only forwarded the last header field of each.  With
this commit, nghttpx forwards all of them if it is configured to do
so.

nghttpx: SNI based backend server selection

nghttpx: Add $tls_sni access log variable

nghttpx: Rename ssl_* log variables as tls_*

The exiting ssl_* log variables still work for backward compatibility.

nghttpx: Fix path matching bug

Previously, if path is empty or path does not start with "/", nghttpx
did not try to match with wildcard pattern.  This commit fixes it.