SSL.packet: Ignore unknown packet types.

RFC 4346 (aka TLS 1.1) 6 recommends that unknown packet types
should be ignored. Note also that the only new packet type that
has been added since TLS 1.1 (PACKET_heartbeat, RFC 6520)
requires that such packets are NOT to be sent during handshakes.
precompile: Support setting a symbol base name.

To avoid conflicting symbol names (especially in program_id.h),
it is often a good idea to prefix the internal symbol names
with the module base name that the cmod file belongs to.
Nettle: Specify a base name to precompile.

The internal C-level symbols generated by precompile are now
prefixed with "Nettle_" or "NETTLE_".
Fix a few renamed symbols.
Nettle: Yet another missed rename.

Fixes broken modes CTR and CCM.
Added block_size() and digest_size() to HMAC
Rename Nettle.Proxy to Nettle.Buffer
Made the code hash block size independent.
SSL.connection: Added missing break.
Generate appropriate extensions on self signed certificates. More WIP on certificate validation.