fix(runsc): isolate xdp flag state

When XDP is not set on the command line, overriding it to ns also changes the shared value used by newly registered flag sets. ToFlags then treats ns as the default and drops --EXPERIMENTAL-xdp=ns.

Give each flag set its own XDP value. Add tests for separate flag sets and for ToFlags after an XDP override.

Tested: //runsc/config:config_test on Linux ARM64. Both new tests fail before the fix.

Refs #15724