Move the restriction to 'rdlen' to the caller.

This creates a new class to represent a TSIG key, containing name,
secret, and algorithm.

The keyring format is changed to be {name : key}, and the methods in
dns.tsigkeyring are updated to deal with old and new formats.

The Message class is updated to use dns.tsig.Key, although (to avoid
breaking existing code), it stores them in the keyring field.

Message.use_tsig() can accept either explicit keys, or keyrings; it will
extract and/or create a key.

dns.message.from_wire() can accept either a key or a keyring in the
keyring parameter.  If passed a key, it will now raise if the TSIG
record in the message was signed with a different key.  If passed a
keyring containing keys (as opposed to bare secrets), it will check that
the TSIG record's algorithm matches that of the key.

The only user of this is the internal _WireReader class, so just pass it
the flag rather than store it on the message.

The sign() and validate() routines took a "first" parameter, which
indicated that this message was the first in a multi-message sequence.
This isn't needed, as it's identical to "not (ctx and multi)".

Remove the parameter from both, as well as the now-unneeded field in the
message object and message.from_wire() parameter.

rcode and EDNS has not previously been activated.  The code is no longer
needed as setting ednsflags to a nonzero value will automatically make
an opt RR and enable EDNS.