bignum: no-op make_string_bn_inplace for LIBGCRYPT

Disable the 'make_string_bn_inplace' helper function for the LIBGCRYPT
build, rather than using '#error' to fail the build completely.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
torture: fix warning for SSH_KEYTYPE_ED25519

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
pki_gcrypt: fix warnings for SSH_KEYTYPE_ED25519

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
pki_gcrypt: fix DSA signature extraction

Fix DSA signature extraction for the LIBGCRYPT build.  Here, the same fix
that was applied to the LIBCRYPTO build for https://red.libssh.org/issues/144
is now adapted for pki_gcrypt.  Additionally, ensure to set the resulting
output sig_blob buffer before returning.

Before this fix, one can observe the failure with the pkd test on a LIBGCRYPT
build as so:

  # ./pkd_hello -i 1 -t torture_pkd_openssh_dsa_dsa_default

After, runs of 10000 back-to-back iterations of the same test are passing.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>