base-files: fix uid/gid auto-enumeration to avoid 16-bit limit

base-files: fix uid/gid auto-enumeration to avoid 16-bit limit

uid/gid range should be limited to 16bit unsigned integer range to
avoid "wraparound" issues with permissions where jffs2
is employed for storage and chown 65536 (first auto-created user)
becomes equivalent to chown 0

Fixes: #13927

Signed-off-by: Thomas Winkler <tewinkler86@gmail.com>
(cherry picked from commit 140b48a9e93909873bee777ca5bdf49fe4f01271)
Link: https://github.com/openwrt/openwrt/pull/15898
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>