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

(cherry picked from commit 3d0a0dda3a7d57bbd4eaf65ba8da0f2a36089c0e)