Be more informative when tap_attach fails

[reynir]

As it is now the error message when attaching to a tap interface fails does not give any explanation as to why it failed.

solo5-spt: Could not attach interface: temp0

I reviewed the code for tap_attach, and in all paths where -1 is returned either errno is set directly or a previous call will have set it. With this patch strerror(errno) is added at the end of the warning:

solo5-spt: Could not attach interface: temp0: Network is down

Now I know to run ip link set temp0 up :-)

[dinosaure]

Seems relatively fine for any platform even if FreeBSD11 and OpenBSD6.8 failed.

[reynir]

Odd. It's unfortunate that there isn't more information on the failed builds. I can't imagine how this change would fail on either platform.