gcrypt: Optionally use internal OAEP padding.

gcrypt: Optionally use internal OAEP padding.

libgcrypt supports OAEP padding since version 1.5.0, which has just
been released. Since want the paraslash gcrypt code to work also
for older gcrypt libraries, we check the library version at runtime
and fall back to the internal OAEP padding code if an old library
was detected.

This patch moves much of the rather ugly OAEP padding code into new
new helper decode_rsa() which reduces to a mere memcpy for newer
gcrypt versions. Much of the old code can be removed once all major
distributions ship libgcrypt-1.5.0 or later.