Suppress -Wunused-result warnings about write() and fwrite().

Tom Lane <tgl@sss.pgh.pa.us>

Commit: aa90e148ca70a235897b1227f1a7cd1c66bc5368
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2011-10-19T01:37:51Z
Releases: 9.2.0
Suppress -Wunused-result warnings about write() and fwrite().

This is merely an exercise in satisfying pedants, not a bug fix, because
in every case we were checking for failure later with ferror(), or else
there was nothing useful to be done about a failure anyway.  Document
the latter cases.

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