pgsql: In psql, use PSQL_PAGER in preference to PAGER, if it's set.

In psql, use PSQL_PAGER in preference to PAGER, if it's set.

This allows the user's environment to set up a psql-specific choice
of pager, in much the same way that we provide PSQL_EDITOR to allow
a psql-specific override of the more widely known EDITOR variable.

Pavel Stehule, reviewed by Thomas Munro

Discussion: https://postgr.es/m/CAFj8pRD3RRk9S1eRbnGm_T6brc3Ss5mohraNzTSJquzx+pmtKA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5e8304fdce2d5c41ef7a648ed0a622480f8f0a07

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 51 ++++++++++++++++++++++-------------------
src/bin/psql/help.c | 4 ++--
src/fe_utils/print.c | 4 +++-
src/interfaces/libpq/fe-print.c | 7 ++++++
4 files changed, 39 insertions(+), 27 deletions(-)