systemctl: systematically rename verb entrypoints verb_xyz()

Let's clean up our function naming a bit, and always name the
verb_xyz(), where the xyz maps to the command line verb as closely as
possible.

No actual code changes, just an attempt to make the systemctl sources a
bit more systematic, and less surprising.

src/systemctl/systemctl-switch-root.h

@@ -1,4 +1,4 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-int switch_root(int argc, char *argv[], void *userdata);
+int verb_switch_root(int argc, char *argv[], void *userdata);