GeezerVison

Since libyang commit CESNET/libyang@be5a07fe0 ("context UPDATE remove binary internal modules", soversion 5.3.4), the internal YANG modules are no longer compiled into the library. ly_ctx_new() loads them from the context search directories, so the caller has to pass ly_yang_module_dir(), and LY_CTX_DISABLE_SEARCHDIRS must not be set before the context exists.

The bindings do neither, so with a recent libyang every Context(...) fails with cannot create context:

ERROR: test_ctx_no_dir (test_context.ContextTest.test_ctx_no_dir)
    raise self.error("cannot create context")
libyang.util.LibyangError: cannot create context

Built against libyang master (soversion 5.5.5), 148 of the 159 unit tests fail on current master. The same failures show up in CI, e.g. in the run for #171.

This PR:

• declares ly_yang_module_dir() and ly_ctx_set_options(),
• always adds the libyang module directory to the search paths,
• applies disable_searchdirs after the context has been created, so the internal modules are still reachable while the option keeps its documented meaning,
• bumps the minimum required libyang soversion to 5.3.4.

With the patch, make lint and all 159 tests pass against libyang master. No test changes were needed: the existing test_ctx_no_dir, test_ctx_disable_searchdirs and test_ctx_using_clb cover the fixed behaviour.