Merge pull request #1 from dapplebeforedawn/add-setsize-for-darwin

Add setsize
Added support for FreeBSD (copy pty_linux.go)
Add pty_unsupported.go file in order to allow projects to import the package and still compile on other os/arch
move `setsize()` back to darwin only
Merge remote-tracking branch 'upstream/master'
add ErrUnsupported for non-Darwin `setsize()`
gofmt