pgsql: Suppress -Wunused-result warnings about write(), again.

Suppress -Wunused-result warnings about write(), again.

Adopt the same solution as in commit aa90e148ca70a235, but this time
let's put the ugliness inside the write_stderr() macro, instead of
expecting each call site to deal with it.  Back-port that decision
into psql/common.c where I got the macro from in the first place.

Per gripe from Peter Eisentraut.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/53c2601a59fc18bd33713162504401f7fec2e2f2

Modified Files
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src/bin/pg_dump/parallel.c | 14 ++++++++++++--
src/bin/psql/common.c      | 26 +++++++++++++++-----------
2 files changed, 27 insertions(+), 13 deletions(-)