[core] server.error-handler new directive for error pages (fixes #2702)

[core] server.error-handler new directive for error pages (fixes #2702)

server.error-handler preserves HTTP status error code when error page
is static, and allows dynamic handlers to change HTTP status code
when error page is provided by dynamic handler.  server.error-handler
intercepts all HTTP status codes >= 400 except when the content is
generated by a dynamic handler (cgi, ssi, fastcgi, scgi, proxy, lua).
The request method is unconditionally changed to GET for the request
to service the error handler, and the original request method is
later restored (for logging purposes).  request body from the
original request, if present, is discarded.

server.error-handler is somewhat similar to server.error-handler-404,
but server.error-handler-404 is now deprecated, intercepts only 404
and 403 HTTP status codes, and returns 200 OK for static error pages,
a source of confusion for some admins.  On the other hand, the new
server.error-handler, when set, will intercept all HTTP status error
codes >= 400.  server.error-handler takes precedence over
server.error-handler-404 when both are set.

NOTE: a major difference between server.error-handler and the
now-deprecated server.error-handler-404 is that the values of the
non-standard CGI environment variables REQUEST_URI and REDIRECT_URI
have been swapped.  Since REDIRECT_STATUS is the original HTTP
status code, REDIRECT_URI is now the original request, and REQUEST_URI
is the current request (e.g. the URI/URL to the error handler).
The prior behavior -- which reversed REQUEST_URI and REDIRECT_URI values
from those described above -- is preserved for server.error-handler-404.

Additionally, REDIRECT_STATUS is now available to mod_magnet, which
continues to have access to request.uri and request.orig_uri.

See further discussion at https://redmine.lighttpd.net/issues/2702
and https://redmine.lighttpd.net/issues/1828

github: closes #36