util: add capsule-util.[ch] with helpers for capsules

For now, there's only a routine for validating capsule names. More will
be added later.

src/shared/capsule-util.c

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "capsule-util.h"
+#include "path-util.h"
+#include "user-util.h"
+
+int capsule_name_is_valid(const char *name) {
+
+        if (!filename_is_valid(name))
+                return false;
+
+        _cleanup_free_ char *prefixed = strjoin("c-", name);
+        if (!prefixed)
+                return -ENOMEM;
+
+        return valid_user_group_name(prefixed, /* flags= */ 0);
+}

src/shared/capsule-util.h

@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+int capsule_name_is_valid(const char *name);

src/shared/meson.build

@@ -140,6 +140,7 @@ shared_sources = files(
         'pkcs11-util.c',
         'plymouth-util.c',
         'pretty-print.c',
+        'capsule-util.c',
         'ptyfwd.c',
         'qrcode-util.c',
         'quota-util.c',