Add GCRYMPI_FLAG_IMMUTABLE to help debugging.
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Description

Add GCRYMPI_FLAG_IMMUTABLE to help debugging.

* src/gcrypt.h.in (GCRYMPI_FLAG_IMMUTABLE): New.
* src/mpi.h (mpi_is_immutable): New macro.
* mpi/mpiutil.c (gcry_mpi_set_flag, gcry_mpi_clear_flag)
(gcry_mpi_get_flag): Implement new flag
(_gcry_mpi_immutable_failed): New.
* mpi/mpiutil.c (_gcry_mpi_clear, _gcry_mpi_free, gcry_mpi_snatch)
(gcry_mpi_set, gcry_mpi_randomize): Act upon the immutable flag.
* mpi/mpi-bit.c (gcry_mpi_set_bit, gcry_mpi_set_highbit)
(gcry_mpi_clear_highbit, gcry_mpi_clear_bit)
(_gcry_mpi_rshift_limbs, gcry_mpi_lshift): Ditto.
* mpi/mpicoder.c (_gcry_mpi_set_buffer): Ditto.

Note that this flag is currently only checked by a few MPI functions.
The reason why we eventually need such a flag is to help implementing
a generic way to retrieve and set ECC parameters without accidentally
changing a curve parameter taken from a list of predefined curves.