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/sysusers/sysusers.c

@@ -207,7 +207,7 @@ static int make_backup(const char *target, const char *x) {
         if (r < 0)
                 return r;
 
-        r = copy_bytes(src, fileno(dst), (off_t) -1, true);
+        r = copy_bytes(src, fileno(dst), (uint64_t) -1, true);
         if (r < 0)
                 goto fail;