Cygwin: net: Make if_nametoindex, etc. consistent with if_nameindex.

Cygwin: net: Make if_nametoindex, etc. consistent with if_nameindex.

Currently, if_nametoindex() and if_indextoname() handle interface names
such as "ethernet_32777", while if_nameindex() returns the names such
as "{5AF7ACD0-D52E-4DFC-A4D0-54D3E6D6B2AC}". This patch unifies the
interface names to the latter.

Fixes: c356901f0d69 ("Rename if_indextoname to cygwin_if_indextoname (analag for if_nametoindex)")
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>