luci-app-wol: replace fs.stat/exec with safe RPC backend, fix ACLs

luci-app-wol: replace fs.stat/exec with safe RPC backend, fix ACLs

fs.stat() and fs.exec() require broad rpcd permissions (ubus file.*) which
prevent luci-app-wol from working for restricted users and expose more
access than necessary.

This introduces a dedicated RPC backend (luci.wol) providing safe stat/exec
wrappers for etherwake and wakeonlan, and simplifies ACLs to only allow
these two RPC calls.

Also adds the missing 'getNetworkDevices' ACL required by DeviceSelect.

Signed-off-by: Martin Devolder <[email protected]>