auth: add PAM support

Handy for small local installations.

Disabled by default because it adds a dependency on the PAM
library.

auth/url.go

@@ -16,6 +16,8 @@ func NewFromURL(authURL string) (AuthProvider, error) {
 		return NewIMAP(u.Host, false), nil
 	case "imaps":
 		return NewIMAP(u.Host, true), nil
+	case "pam":
+		return NewPAM()
 	default:
 		return nil, fmt.Errorf("no auth provider found for %s:// URL", u.Scheme)
 	}