Use appropriate request for ioctl() function to access the interface flags (thanks to Peter Koletzki for the report)

[ming]

When using SIOCGIFCONF ifreq struct would be filled with the interface address rather than the flags of the interface. Therefore any test against the flags of this interface are not working at the moment.

For gaining access to the flags, an additional socketctrl SIOCGIFFLAGS should be used.

[ming]

In 4326:

Fixed #1613: Use appropriate request for ioctl() function to access the interface flags

[ming]

In 4327:

Re #1613: Remove unused variable

[bennylp]

In r4355: fixed bugs in the r4326 above:

• the socket was already closed when ioctl() is called
• the 'iff' structure was not initialized and did not contain the interface name

[riza]

In 4403:

Re #1613: backported to 1.x