Add OpenBSD support

[fd0]

This commit contains the following changes:

• Rename *_freebsd.go -> *_bsd.go
• Add build tag to *_bsd.go to build for FreeBSD and OpenBSD
• Replace syscall.EPROTO with syscall.EIO, as the latter is the more portable error code. This change was inspired by sshfs, which introduced it in 2005 (release 1.4).

[fd0]

This doesn't work yet, the fuse device on OpenBSD is /dev/fuse0 instead of /dev/fuse, and mount_fusefs doesn't exist. I'll investigate and push more commits.

[tv42]

Thanks for all the effort.

[fd0]

This somehow doesn't work, the mount syscall fails with a strange error, and I cannot find out what's wrong. I've pushed my last state so that someone doesn't have to start from the beginning again.

[ramrunner]

Hey. thanks for trying to make this to work. i actually have managed to not get an error from the mount syscall using most of your code. The problem i have is that right after mount we are in initmount() which issues a ReadRequest on the fd. The read syscall on that returns 0. @tv42 i'm not familiar with fuse internals but how is this supposed to work? what request can i read from the fuse device initially without it being aware of anything? looking at what libfuse on openbsd does the codepath is quite similar but they just set a kqueue there after mount to be notified for read events from the device. any input that would help us proceed? thanks!
(ps. @fd0 where you getting "device not configured"? if yes it might be because you already have called mount on that dir. on a new dir i don't get a mount error)

[tv42]

@ramrunner The first messages exchanged over /dev/fuse are InitRequest and InitResponse. At least they are everywhere else except OpenBSD!

[qbit]

For anyone that starts working on this again, I have an updated branch (basically the diff from this PR updated to work with 371fbbd: https://github.com/qbit/fuse

go build works fine. go test results in a device not configured directory mounted in /tmp