Improved adapter name generation when no adapter name is specified.

Improved adapter name generation when no adapter name is specified.

Try to find an adapter name that is not already used in the system by
appending a monotonically increasing integer to the hostname that is
used as a default name.

This works around wintun's weird behaviour of renaming existing adapters
without preventing two or more instances of openconnect to connect to
the same VPN host (without explicity specifying an interface name), or
otherwise messing with user's network adapters.

Signed-off-by: Marios Paouris <mspaourh@gmail.com>