introduce custom HTTP status pages [NEEDS WORK]

introduce custom HTTP status pages [NEEDS WORK]

as an additional bonus, child processes of `httpd.execline` now inherit
the environment variables for the parsed start line and headers; this
allows `http-error-response.execline` to make use of the host supplied
by the client request with minimal changes.

note that `Content-Length` is currently not supplied anymore for header
responses (functional, but the HTTP/1.1 standard says you “SHOULD” use
that header); this needs fixing.

also note that `http-error-response.execline` currently forces error
status pages to use a `Content-Type` of
`application/xhtml+xml; charset=utf-8`; surely we should allow the user
to override this. (perhaps make status code *folders*, and use
constituent files `Content-Type`, perhaps `extra_headers`, and of course
`message_body`, in that directory? this interface would be less than
ideal though)