tree-wide: extend verbs functions with extra per-verb data parameter

We often have a pattern where the same verb function is used for
multiple actions. This leads to an antipattern where we figure out what
action needs to be taken from argv[0] multiple times: often once in
parse_argv() to figure out what options are allowed, then once again
implicitly in dispatch_verb(), and then again in the action verb itself.
Let's allow passing a parameter into the verb to simplify this.

This matches a pattern we have in conf-parser.h, where we have both
void *userdata (more global) and void *data (per-config item). Here,
I opted for uintptr_t userdata. It seems that most of the time we'll
want to just pass an enum value. This works OK with no casts. I also
tried a void* and union. In both cases, much more boilerplate is needed:
either a cast or a macro to help avoid compiler warnings. uintptr_t
seems generic enough to cover foreseeable usecases with no fuss.

This is a noop refactoring. See next commit for an example.

src/backlight/backlight.c

@@ -555,7 +555,7 @@ static int device_new_from_arg(const char *s, sd_device **ret) {
         return 1; /* Found. */
 }
 
-static int verb_load(int argc, char *argv[], void *userdata) {
+static int verb_load(int argc, char *argv[], uintptr_t _data, void *userdata) {
         _cleanup_(sd_device_unrefp) sd_device *device = NULL;
         unsigned max_brightness, brightness, percent;
         bool clamp;
@@ -605,7 +605,7 @@ static int verb_load(int argc, char *argv[], void *userdata) {
         return 0;
 }
 
-static int verb_save(int argc, char *argv[], void *userdata) {
+static int verb_save(int argc, char *argv[], uintptr_t _data, void *userdata) {
         _cleanup_(sd_device_unrefp) sd_device *device = NULL;
         _cleanup_free_ char *path = NULL;
         unsigned max_brightness, brightness;