daemon: Add '--timeout' and '--max-silent-time'.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_TIMEOUT)
(GUIX_OPT_MAX_SILENT_TIME): New macros.
* nix/nix-daemon/guix-daemon.cc (options): Add '--timeout' and
'--max-silent-time'.
(parse_opt): Honor them.
* tests/guix-daemon.sh: Add test.
* doc/guix.texi (Invoking guix-daemon): Document the options.
(Common Build Options): Properly describe default
timeout/max-silent-time value.  Add cross-ref to "Invoking
guix-daemon".