pgsql: Add \warn command to psql.

Add \warn command to psql.

This is like \echo except that the text is sent to stderr not stdout.

In passing, fix a pre-existing bug in \echo and \qecho: per documentation
the -n switch should only be recognized when it is the first argument,
but actually any argument matching "-n" was treated as a switch.
(Should we back-patch that?)

David Fetter (bug fix by me), reviewed by Fabien Coelho

Discussion: https://postgr.es/m/20190421183115.GA4311@fetter.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/02e95a5049f7933cbde1dacf401604ea3fc02aa5

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 20 ++++++++++++++++----
src/bin/psql/command.c | 9 ++++++---
src/bin/psql/help.c | 7 ++++---
src/bin/psql/tab-complete.c | 2 +-
src/test/regress/expected/psql.out | 19 +++++++++++++++++++
src/test/regress/sql/psql.sql | 16 ++++++++++++++++
6 files changed, 62 insertions(+), 11 deletions(-)