Currently `Write` returns the number of ciphertext bytes written.
According to the docs for io.Writer, Write should return the amount
of bytes consumed from the input.
```
// Write writes len(p) bytes from p to the underlying data stream.
// It returns the number of bytes written from p (0 <= n <= len(p))
// and any error encountered that caused the write to stop early.
// Write must return a non-nil error if it returns n < len(p).
// Write must not modify the slice data, even temporarily.
Write(p []byte) (n int, err error)
```
Fixes#4126
Signed-off-by: Maisem Ali <maisem@tailscale.com>
With this change, the client can obtain the initial handshake message
separately from the rest of the handshake, for embedding into another
protocol. This enables things like RTT reduction by stuffing the
handshake initiation message into an HTTP header.
Similarly, the server API optionally accepts a pre-read Noise initiation
message, in addition to reading the message directly off a net.Conn.
Updates #3488
Signed-off-by: David Anderson <danderson@tailscale.com>
