gpg: Fix segv with RSA_S keys.

* g10/misc.c (pubkey_get_npkey, pubkey_get_nskey)
(pubkey_get_nsig, pubkey_get_nenc): Map all RSA algo ids to
GCRY_PK_RSA.

The problem is that Libgcrypt has no more support for the alternate
RSA ids and thus if asking for the number of parameters, they will
return zero. Now, this leads to packing the key parameters into an
opaque MPI but because the algorithm id is actually known to GPG, it
assumes valid RSA parameters.

An example key with RSA_S is 0x5434509D.