nghttpx: Add frontend-tls parameter to backend to require client TLS

LibreSSL removed PSK
Signed-off-by: Bernard Spil <brnrd@FreeBSD.org>

Write integer to log buffer directly to improve efficiency.  Remove
unused function templates.  Use [first, last) style arguments for
copy() function templates.

Previously, when parsing HTML links, we only take into account
overridden host.  But we actually need more variables to consider.  In
this commit, we take into account overridden scheme, host, and port to
parse HTML links.

Fix authority for --get-assets if IP adress is used in conjunction with user-defined :authority header

--accesslog-write-early option is analogous to HAProxy's logasap.  If
used, nghttpx writes access log when response header fields are
received from backend rather than when request transaction finishes.