sd-varlink: make our internal Varlink API public as sd-varlink.[ch]

It's time. sd-json was already done earlier in this cycle, let's now
make sd-varlink public too.

This is mostly just a search/replace job of epical proportions.

I left some functions internal (mostly IDL handling), and I turned some
static inline calls into regular calls.

src/shared/varlink-io.systemd.MountFileSystem.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "varlink-idl.h"
+#include "sd-varlink-idl.h"
 
-extern const VarlinkInterface vl_interface_io_systemd_MountFileSystem;
+extern const sd_varlink_interface vl_interface_io_systemd_MountFileSystem;