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/nspawn/nspawn.c

@@ -3601,7 +3601,7 @@ finish:
 
         /* Try to flush whatever is still queued in the pty */
         if (master >= 0)
-                (void) copy_bytes(master, STDOUT_FILENO, (off_t) -1, false);
+                (void) copy_bytes(master, STDOUT_FILENO, (uint64_t) -1, false);
 
         loop_remove(loop_nr, &image_fd);