Turn security.pam.services into an attribute set

That is, you can say

  security.pam.services.sshd = { options... };

instead of

  security.pam.services = [ { name = "sshd"; options... } ];

making it easier to override PAM settings from other modules.
sshd: Remove the usePAM option

Sshd *must* use PAM because we depend on it for proper session
management.  The original goal of this option (disabling password
logins) can also be implemented by removing pam_auth authentication
from sshd's PAM service.