Include utun device number in utun error messages

Include utun device number in utun error messages

For lack of a better API (or knowledge about a better API) we try to
open utun devices on macOS by trying utun0 to utun255 and use the
first one that works. On my Mac I have already 4 devices that
do nothing but are just there and another VPN connection resulting in a
number of error messages. This explicitly  shows in the log that we
tried the devices instead of some unspecific error.

This changes the log from:

Opening utun (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opened utun device utun5

to

Opening utun0 failed (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun1 failed (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun2 failed (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun3 failed (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opening utun4 failed (connect(AF_SYS_CONTROL)): Resource busy (errno=16)
Opened utun device utun5

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Feature-ACK-by: "Jonathan K. Bullard" <jkbullard@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200725235023.22441-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20590.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>