Use EVP_PKEY_dup instead of manually incrementing the refcount.

Reference counting should be internal to the type, otherwise callers need to
know which lock to use.

Change-Id: If4d805876a321ef6dece115c805e605584ff311e
Reviewed-on: https://boringssl-review.googlesource.com/1414
Reviewed-by: Adam Langley <agl@google.com>