* guix/scripts/download.scm (%default-options): Add 'hash-algorithm'. (show-help, %options): Add "--hash". (guix-download): Honor it. * guix/scripts/hash.scm (%default-options): Add 'hash-algorithm'. (show-help, %options): Add "--hash". (guix-hash): Honor it. * tests/guix-hash.sh: Test '-H sha512'. * doc/guix.texi (Invoking guix download): Document it. (Invoking guix hash): Document it.