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.c

@@ -11,7 +11,7 @@
 #include "systemctl-util.h"
 #include "systemctl.h"
 
-int switch_root(int argc, char *argv[], void *userdata) {
+int verb_switch_root(int argc, char *argv[], void *userdata) {
         _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
         _cleanup_free_ char *cmdline_init = NULL;
         const char *root, *init;