Fixes <http://bugs.gnu.org/17312>.
* guix/scripts/authenticate.scm (guix-authenticate): Add calls to
'set-port-encoding!' and 'set-port-conversion-strategy!'. Wrap body
in 'with-fluids' form that sets '%default-port-encoding' and
'%default-port-conversion-strategy'.
* tests/guix-authenticate.sh: Add test.
* tests/pk-crypto.scm ("hash corrupt due to restrictive locale
encoding"): Add reference to bug.
* guix/scripts/authenticate.scm (guix-authenticate): Add clauses
for ("rsautl" "-sign" "-inkey" key) and ("rsautl" "-verify" "-inkey" _
"-pubin").
* tests/guix-authenticate.sh (hash): Add test using -sign and -verify in
a pipeline.
