ssl: support ALPN

Heavily based on the existing NPN wrapping code. Naming of public
functions is identical to the NPN ones with `s/npn/alpn/` applied to
prevent devs from needing to remember 2 names (and to let my copy the
npn tests and perform the subistution to generate the apln tests).

It might make sense to (at some point) use macros or a trait to cut down
the duplication.