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-kill.c

@@ -6,7 +6,7 @@
 #include "systemctl-util.h"
 #include "systemctl.h"
 
-int kill_unit(int argc, char *argv[], void *userdata) {
+int verb_kill(int argc, char *argv[], void *userdata) {
         _cleanup_strv_free_ char **names = NULL;
         char *kill_who = NULL, **name;
         sd_bus *bus;