auth: add PAM support

Handy for small local installations.

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

auth/pam_stub.go

@@ -0,0 +1,11 @@
+//go:build !pam
+
+package auth
+
+import (
+	"errors"
+)
+
+func NewPAM() (AuthProvider, error) {
+	return nil, errors.New("PAM support is disabled")
+}