win32 fixes

For application to HEAD, following community review.

* Most changes are to fix warnings issued when compiling win32
* removed a few redundant defines
* get_user_name safe under win32
* rationalized pipe read EINTR for win32
* changed all backend instances of sleep() to pg_usleep
- except for the SLEEP_ON_ASSERT in assert.c, as it would exceed a
32-bit long [Note to patcher: If a SLEEP_ON_ASSERT of 2000 seconds is
acceptable, please replace with pg_usleep(2000000000L)]

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