Fix platform specific code for FreeBSD

[dveeden]

Fix build on FreeBSD

There is no syscall.Dup3 on FreeBSD. This leads to:

% go build
# github.com/ngaut/log
./crash_unix.go:16:11: undefined: syscall.Dup3

This is with FreeBSD 13. and Go 1.19.1

As the macOS code already uses Dup2 instead of Dup3 and the generic UNIX code doesn't set any flags it looks like it might be best to use Dup2 for all platforms.

[dveeden]

I also filed this: golang/go#55935