The only point format that we ever support is uncompressed, which the
RFC says implementations MUST support. The TLS 1.3 and Curve25519
forecast is that point format negotiation is gone. Each curve has just
one point format and it's labeled, for historial reasons, as
"uncompressed".

Change-Id: I8ffc8556bed1127cf288d2a29671abe3c9b3c585
Reviewed-on: https://boringssl-review.googlesource.com/5542
Reviewed-by: Adam Langley <agl@google.com>