Static variables with __thread instead/POSIX

To make mosh multithreaded I had to transform all of the static variables into per thread ones. I used to routes to do this, using POSIX thread variables as much as I could as it is compatible with more iOS versions. Unfortunately some of them couldn't be done in this way and where triggered at the compiler level with __thread.

It would be great to remove this two implementations and either go with one for everything, or even better, fix mosh from the root.

The issue names no files, tests, or entry points. Start by inventorying static variables and locating both POSIX thread-variable and __thread implementations, then determine whether a single approach can support the required iOS versions; done means removing the duplicated mechanisms or addressing the underlying threading design.

Written by the indexing model from the issue text.