gcrypt: Add support for RFC4716 private keys.

gcrypt: Add support for RFC4716 private keys.

This teaches the gcrypt backend to parse private keys generated
with ssh-keygen -m RFC4716. Support for the openssl backend will be
implemented in a subsequent patch. The new find_openssh_bignum_offset()
is independent of the gcrypt API. We put this function into
crypt_common.c rather then into gcrypt.c so that openssh.c can also
use it.