Test readx/writex (now renamed).

Renamed readx/writex to ReadFdExactly/WriteFdExactly respectively.
These read/write a full fixed-size buffer. If the whole buffer cannot
be read/written, these functions return an error.

Rename write_string to WriteStringFully.

Move the TEMP_FAILURE_RETRY definition in sysdeps.h out of the
!Windows section. It seems Windows won't actually interrupt a call,
but it's easier to just define it than to #ifdef each call.

Change-Id: Ia8ddffa2a52764a2f9a281c96c937660e002b9b9