* Add OpenSSL as supported licence type

Adds special handling for the OpenSSL licensed files under BoringSSL.
Specifically, some of the ARM assembly files in the latest Dart SDK are
dual-licensed with OpenSSL.

* Support blocks that ref by type with no copyright

The upcoming Dart SDK includes files (e.g., sha256-armv4.S) with a block
that specifies teh code is dual-licenced under OpenSSL, but doesn't
include a copyright date of its own.