tree-wide: never use the off_t unless glibc makes us use it

off_t is a really weird type as it is usually 64bit these days (at least
in sane programs), but could theoretically be 32bit. We don't support
off_t as 32bit builds though, but still constantly deal with safely
converting from off_t to other types and back for no point.

Hence, never use the type anymore. Always use uint64_t instead. This has
various benefits, including that we can expose these values directly as
D-Bus properties, and also that the values parse the same in all cases.

src/libsystemd/sd-bus/busctl.c

@@ -1823,20 +1823,20 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_SIZE: {
-                        off_t o;
+                        uint64_t sz;
 
-                        r = parse_size(optarg, 1024, &o);
+                        r = parse_size(optarg, 1024, &sz);
                         if (r < 0) {
                                 log_error("Failed to parse size: %s", optarg);
                                 return r;
                         }
 
-                        if ((off_t) (size_t) o !=  o) {
+                        if ((uint64_t) (size_t) sz !=  sz) {
                                 log_error("Size out of range.");
                                 return -E2BIG;
                         }
 
-                        arg_snaplen = (size_t) o;
+                        arg_snaplen = (size_t) sz;
                         break;
                 }