Internal API change to accommodate MatrixSSH users.
Functional changes
Ignore TLS extensions sent with SSL 3.0 ClientHello. Thunderbird sends these extensions if negotiating down from a TLS connection, even though they are meaningless.
Enhanced the parsing of the Key Usage certificate extension.
Bug fixes and optimizations
Assure file reads into memory are NULL terminated. This was an issue flagged by Valgrind that doesn't present a problem in practice.
