ssh-keygen: Generate Ed25519 keys when invoked without arguments

Ed25519 keys are convenient because they're much smaller, and the next
OpenSSH release (9.5) will switch to them by default. Apply the change
to FreeBSD main now, to help identify issues as early as possible.

Reviewed by: kevans, karels, des
Relnotes: Yes
Obtained from: OpenBSD 9de458a24986
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41773