Over time the amount of custom Go patches reduced, and tris got less
tangled to the underlying Go. Finally sever the link.
This allows more flexibility in what base Go is used (the system one),
doesn't require coordinating two repositories, and simplifies the black
magic considerably.
Make sure to use tris with Go 1.9.X.
Force boringssl client and server to use draft 22 instead of draft 18.
Other clients (tstclnt from NSS, picotls) support only draft 22.
Disable mint, it only supports draft 21.
