Add an authproto_pam helper that communicates with PAM.

This is now used by auth_pam_x11 as sole backend. It can be made
setuid/setgid where necessary to authenticate local users (e.g. on
FreeBSD).

Also renames auth_pam_x11 to auth_x11, and moves auth_htpasswd and
auth_pamtester to authprotos that can be used with auth_x11.

Will fix #43.