tree-wide: some more [static] related fixes

let's add [static] where it was missing so far

Drop [static] on parameters that can be NULL.

Add an assert() around parameters that have [static] and can't be NULL
hence.

Add some "const" where it was forgotten.

src/systemctl/systemctl.c

@@ -3021,6 +3021,8 @@ static enum action verb_to_action(const char *verb) {
 static const char** make_extra_args(const char *extra_args[static 4]) {
         size_t n = 0;
 
+        assert(extra_args);
+
         if (arg_scope != UNIT_FILE_SYSTEM)
                 extra_args[n++] = "--user";