mpi: Clear immutable flag on the result of gcry_mpi_set.

* mpi/mpiutil.c (gcry_mpi_set): Reset immutable and const flags.
* tests/mpitests.c (test_const_and_immutable): Add a test for this.

gcry_mpi_set shall behave like gcry_mpi_copy and thus reset those
special flags. Problem reported by Christian Grothoff.

• Signed-off-by: Werner Koch <wk@gnupg.org>