gpg: add EdDSA algorithm
ui: update algorithm display

"Key length" doesn't make sense with ECC keys, so change the
human-friendly algorithm representation to show something sensible
for both EC and conventional keys.
link agent socket before importing keys

GnuPG 2.1 starts an agent in the temporary homedir (if there isn't
one already) when importing keys, and it doesn't have access to the
private key material.

Avoid this situation by linking the temporary homedir to the primary
agent socket before importing keys.
todo: add some items