Added ldap and pam authentication

internal/pamauth/stub.go

@@ -0,0 +1,21 @@
+//go:build !pam
+
+// Package pamauth authenticates a username/password pair against the host's PAM
+// stack. This is the stub compiled into the default fully-static
+// (CGO_ENABLED=0) build, which has no PAM/libpam linkage: Authenticate always
+// reports PAM is unavailable. Build with `make backend-pam` (CGO_ENABLED=1
+// -tags pam) to get the real implementation in pam.go.
+package pamauth
+
+import "errors"
+
+// Available reports whether this build includes PAM support. False here.
+const Available = false
+
+// ErrUnavailable is returned by Authenticate because this build lacks PAM.
+var ErrUnavailable = errors.New("pamauth: PAM support not compiled in (rebuild with: make backend-pam)")
+
+// Authenticate always fails in the non-PAM build.
+func Authenticate(service, username, password string) error {
+	return ErrUnavailable
+}