core: Improve mailbox only uid handling

* src/key.c (_gpgme_key_append_name): Set email and remove name
for uid only keys.

If we have a name and no email but the name can be parsed as
an address we now treat the address as email and remove the name.

This fixes downstream users that rely on email to show email
addresses and don't expilicity handle this case.

E.g. A userid foo@example.com was:
uid->name = "foo@example.com"
uid->email = ""
uid->address = "foo@example.com"

It is now:
uid->name = ""
uid->email = "foo@example.com"
uid->address = "foo@example.com"