hostapd: Replace UDP ctrl_iface global cookies with per-instance ones

hostapd: Replace UDP ctrl_iface global cookies with per-instance ones

The cookie values for UDP control interface commands was defined as a
static global array. This did not allow multi-BSS test cases to be
executed with UDP control interface. For example, after
    hapd1 = hostapd.add_bss(apdev[0], ifname1, 'bss-1.conf')
    hapd2 = hostapd.add_bss(apdev[0], ifname2, 'bss-2.conf')

hapd1->ping() did not work.

Move those cookie values to per-instance location in struct
hapd_interfaces and struct hostapd_data to fix this.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@gmail.com>