Add tailscale_enable_funnel_to_localhost_plaintext_http1 to tailscale.h

[yagil]

Adds a way to enable a funnel from libtailscale.

[bradfitz]

Can you squash your commits in the branch together, down to 1 commit?

[bradfitz]

@raggi, you cool with this API and name?

This gets it out of the way of something more general, like sent in #28.

[yagil]

Can you squash your commits in the branch together, down to 1 commit?

done

[raggi]

@raggi, you cool with this API and name?

This gets it out of the way of something more general, like sent in #28.

Yeah absolutely. We might want to take a struct approach, but we'd then need to decide about the whole enum ffi debacle. This SGTM.

[bradfitz]

Please squash. We have "Squash & Merge" disabled on this repo.

[yagil]

Please squash. We have "Squash & Merge" disabled on this repo.

Done