Signals continued

This gives us consistency, but it doesn't entirely solve the syscall restart problem
Reads and writes may still return early
Other syscalls aren't guaranteed to complete either

It is still necessary to retry important calls on failure

For read() and write() we use the atomicio() wrapper
Automatically restarts reads or writes, updating buffer offset and transfer count
Also returns size_t (unsigned), to make our code easier to rid of signed vs. unsigned bugs

Downside: parts of the application block
Makes turning ssh into a library more difficult