library: Add facility to register custom init/deinit functions

These can be linked into the application to do initialization/cleanup
without having to modify the source code.

src/libstrongswan/library.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2010-2018 Tobias Brunner
+ * Copyright (C) 2010-2025 Tobias Brunner
  * Copyright (C) 2008 Martin Willi
  *
  * Copyright (C) secunet Security Networks AG
@@ -285,6 +285,24 @@ bool library_init(char *settings, const char *namespace);
  */
 void library_deinit();
 
+/**
+ * Custom function called during init/deinit of a library.
+ *
+ * @param init			TRUE during init, FALSE during deinit
+ * @return				FALSE if an error occurred and init should fail
+ */
+typedef bool (*library_init_t)(bool init);
+
+/**
+ * Register a custom init function that's called at the end of library_init()
+ * and the start of library_deinit().
+ *
+ * To be called from __attribute__((constructor)) functions.
+ *
+ * @param init				init function
+ */
+void library_init_register(library_init_t init);
+
 /**
  * Library instance, set after library_init() and before library_deinit() calls.
  */