pkg: finish adding the ECC signer and signature type bits

Signature types need to be parsed out of the key/signature information
that we are presented with from the files we download. We use that to
understand whicher signer we need to dispatch to.

The ECC signer is more-or-less lifted from pkg(8), with some changes to
slim it down for pkg(7).

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D48108