Simplify Windows client to react to Ctrl-C always

Simplify Windows client to react to Ctrl-C always

When select() gets interrupted by SIGINT while a handler is active
without SA_RESTART, it returns immediately with EINTR.  w32_select()
did that only while it waited for standard input to become ready for
reading.  This isn't the case when:

* The client has already received EOF on standard input.  But then the
  action is SIG_DFL, so there was no problem.

* Reading standard input is suspended until the server drains the
  input buffer.  Then reaction to Ctrl-C got delayed until the socket
  got ready, and w32_select() returned normally.  Harmless, because
  the reaction merely appends to the input buffer.

Change w32_select() to match select()'s behavior anyway.