pg_arch.c call to sleep()

We have the following warning on Windows:

pgarch.c:349: warning: implicit declaration of function `sleep'

To fix it we could include the right header (which appears to be 
<stdlib.h> in the Windows/Mingw case), or we could replace the call by a 
call to pg_usleep().

I'm inclined to do the latter, on grounds of consistency, but want to be 
sure that it will get the signal handling right.

Unless I hear squawks of objection I will supply a patch for that soon.

cheers

andrew