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-uki.h

@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+int verb_kernel_identify(int argc, char *argv[], void *userdata);
+int verb_kernel_inspect(int argc, char *argv[], void *userdata);