This change exposes the functions needed to support arbitrary elliptic
curve groups. The Java API[1] doesn't allow a provider to only provide
certain elliptic curve groups. So if BoringSSL is an ECC provider on
Android, we probably need to support arbitrary groups because someone
out there is going to be using it for Bitcoin I'm sure.

Perhaps in time we can remove this support, but not yet.

[1] https://docs.oracle.com/javase/7/docs/api/java/security/spec/ECParameterSpec.html

Change-Id: Ic1d76de96f913c9ca33c46b451cddc08c5b93d80
Reviewed-on: https://boringssl-review.googlesource.com/4740
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>