Add OpenBSD build support

[byteonabeach]

Building Zoraxy on OpenBSD 7.8 (amd64) fails because the sshprox package uses symbols UseWinTTY and gotty that are only defined in files with build tags that exclude OpenBSD.

The error is:
mod/sshprox/sshprox.go:67:5: undefined: UseWinTTY
mod/sshprox/sshprox.go:109:5: undefined: UseWinTTY
mod/sshprox/sshprox.go:148:20: undefined: gotty
mod/sshprox/sshprox.go:155:22: undefined: gotty
mod/sshprox/sshprox.go:209:5: undefined: UseWinTTY
mod/sshprox/utils.go:24:5: undefined: UseWinTTY
mod/sshprox/utils.go:36:12: undefined: gotty

I add a single file, src/mod/sshprox/sshprox_openbsd.go, with the build constraint //go:build openbsd. It defines UseWinTTY = false and provides an embedded filesystem for the gotty directory. No existing files are modified.

After the change the package compiles and the binary links on OpenBSD. The gotty executable for OpenBSD is not embedded, so the SSH proxy feature will still return "platform not supported" at runtime. This PR only addresses the build breakage.

[tobychui]

Hey @byteonabeach

This PR only addresses the build breakage.

Yes, this is exactly what I want to say. The better way to do this is just set UseWinTTY to true and use the Zoraxy internal tty terminal feature instead. Maybe you can reference sshprox/embed_windows_amd64.go and modify the OpenBSD version accordingly?

[tobychui]

As previous comment, please use the internal WinTTY feature instead.

[tobychui]

Well since OpenBSD is not in our supported list and this only solved compile issue (function still unusable after this patch), I think I better close this. Currently there is no plan to support OpenBSD officially, but feel free to maintain a community port for this if needed.