The Utsname members are only 32 bytes on Dragonfly and the syscall
returns ENOMEM in case the value is longer than that. Like uname(3)
on Dragonfly, handle this case gracefully and just truncate the
value.

Change-Id: If617af1b6831cff6d4245f498dad9f264b8fd118
Reviewed-on: https://go-review.googlesource.com/82155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>