[mod_fastcgi] use http_response_xsendfile() (fixes #799, fixes #851, fixes #2017...

[mod_fastcgi] use http_response_xsendfile() (fixes #799, fixes #851, fixes #2017, fixes #2076)

handle X-Sendfile and X-LIGHTTPD-send-file w/ http_response_xsendfile()
  if host is configured ( "x-sendfile" = "enable" )

Note: X-Sendfile path is url-decoded for consistency, like X-Sendfile2
      (response headers should be url-encoded to avoid tripping over
       chars allowed in filesystem but which might change response
       header parsing semantics)

Note: deprecated: "allow-x-send-file";         use "x-sendfile"
Note: deprecated: X-LIGHTTPD-send-file header; use X-Sendfile header
Note: deprecated: X-Sendfile2 header;          use X-Sendfile header
For now, X-Sendfile2 is still handled internally by mod_fastcgi.

Since http_response_send_file() supports HTTP Range requests,
X-Sendfile2 is effectively obsolete.  However, any code, e.g. PHP,
currently using X-Sendfile2 is probably manually generating 206 Partial
Content status and Range response headers.  A future version of lighttpd
might *remove* X-Sendfile2.  Existing code should be converted to use
X-Sendfile, which is easily done by removing all the special logic
around using X-Sendfile2, since the 206 Partial Content status and Range
response headers are handled in http_response_send_file().

x-ref:
  "mod_fastcgi + X-Sendfile -> mod_staticfile"
  https://redmine.lighttpd.net/issues/799
  "Feature Request: New option "x-send-file-docroot""
  https://redmine.lighttpd.net/issues/851
  "X-Sendfile handoff to mod-static-file in 1.4.x"
  https://redmine.lighttpd.net/issues/2017
  "X-sendfile should be able to set content-type"
  https://redmine.lighttpd.net/issues/2076