Move bootctl to its own source subdirectory

It's been split into a bunch of files and deserves its own subdirectory
similarly to systemctl.

src/bootctl/bootctl-status.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "sd-varlink.h"
+
+int verb_status(int argc, char *argv[], void *userdata);
+int verb_list(int argc, char *argv[], void *userdata);
+int verb_unlink(int argc, char *argv[], void *userdata);
+
+int vl_method_list_boot_entries(sd_varlink *link, sd_json_variant *parameters, sd_varlink_method_flags_t flags, void *userdata);