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.

Update nghttp2_session.c

Add missing free call on error in inflight_settings_new().

Support specifying stream priority via session::submit()

This commit adds 2 new options to handle X-Forwarded-Proto header
field.  The --no-add-x-forwarded-proto option makes nghttpx not to
append X-Forwarded-Proto value.  The
--no-strip-incoming-x-forwarded-proto option prevents nghttpx from
stripping the header field from client.

Previously, nghttpx always strips incoming header field, and set its
own header field.  This commit preserves this behaviour, and adds
additional knobs.

This reverts commit 8c0b2c68.

This reverts commit 6aa581d2.

This reverts commit 82b326e6.