deprecate key_load_private_pem() and sshkey_load_private_pem()

deprecate key_load_private_pem() and sshkey_load_private_pem()
interfaces. Refactor the generic key loading API to not require
pathnames to be specified (they weren't really used).

Fixes a few other things en passant:

Makes ed25519 keys work for hostbased authentication (ssh-keysign
previously used the PEM-only routines).

Fixes key comment regression bz#2306: key pathnames were being lost as
comment fields.

ok markus@