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
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/ec5622351208989620b6563b7890922e28e65e7b

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