Use the pre-unwind-handler of a catch clause instead of nesting
with-throw-handler inside a catch clause.

* src/web/server/fiberized.scm (with-ignored-disconnects): Remove it,
(client-loop): replace "with-ignored-disconnects" with a catch clause.

EPIPE and ECONNRESET errors are already ignored when thrown by "sendfile" and
"put-bytevector" procedures. It turns out "peek-char" can also cause such
errors. So ignore such errors within the whole procedure.

* src/web/server/fiberized.scm (client-loop): Wrap the whole procedure inside
"with-ignored-disconnects" instead of wrapping individually "sendfile" and
"put-bytevector" calls.

* src/web/server/fiberized.scm (with-ignored-disconnects): New macro
factorizing the catch clause ignoring client disconnection related errors. Use
it for both "sendfiles" and "put-bytevector" procedures.

* src/web/server/fiberized.scm (client-loop): Catch EPIPE and ECONNRESET
errors than can be thrown if the client disconnects prematurely.

* src/cuirass/http.scm (respond-file): Send the file name as 'x-raw-file
header argument, instead of the raw file content,
(respond-gzipped-file): ditto. Also set 'content-disposition header.
* src/web/server/fiberized.scm (strip-headers, with-content-length): New procedures,
(client-loop): Check if 'x-raw-file is set. If it's the case, use sendfiles to
send the given file. Otherwise, keep the existing behaviour and send directly
the received bytevector.

* src/web/server/fiberized.scm (socket-loop): Remove "HTTP connection"
message logging.

* src/web/server/fiberized.scm (socket-loop): Add 'log-message' call.
* src/cuirass/http.scm (url-handler): Likewise.

* src/web/server/fiberized.scm: New file.
* Makefile.am (dist_pkgmodule_DATA): Add it.
* src/cuirass/http.scm (run-cuirass-server): Use it.