Use pipe to communicate between client and server at startup

[ahedberg]

Fixes #1142

[achernya]

Can we augment the git commit description with how this commit works? The PR description won't show up in the git log

[cgull]

Fixes #1142

I don't think this fixes anything? In the prior mosh-server code, that read/write on the pty are before any user data passes across it, the newline should never be seen by the user. The problems with excess newlines are in mosh.pl, at least in my experience.

I am concerned about the additional complexity added by this commit. At the very least, CLOEXEC should be set on the pipe fds.

[achernya]

that read/write on the pty are before any user data passes across it, the newline should never be seen by the user.

Empirically, it is being seen by the user.

[achernya]

@ahedberg I fixed the CI config, I believe if you rebase and push CI should be clean.