Normalize 'usage' argument name

Now, all instances of the 'usage' argument are called 'usage'
(previously, some were called 'cred_usage').
Normalize away from 'desired_' argument names

All argument names of the form `desired_foo` have been converted
to simply `foo`, with the exception of `desired_output_size` for
clarity.
Normalize 'lifetime' argument name

All instances of the argument 'ttl' have been converted to 'lifetime'
(including compound argument names with 'ttl', such as 'init_ttl',
which became 'init_lifetime').

Additionally, instances of 'initiator_lifetime' and 'acceptor_lifetime'
have been converted into 'init_lifetime' and 'accept_lifetime'.
Normalize 'mech' argument name

All instances of the 'mech_type' and 'mech_types' arguments have
been converted into the 'mech' and 'mechs' arguments, respectively.

Additionally, the relevant properties of the named tuples have been
updated as well.