pgsql: Add a \getenv command to psql.

Add a \getenv command to psql.

\getenv fetches the value of an environment variable into a psql
variable. This is the inverse of the \setenv command that was added
over ten years ago. We'd not seen a compelling use-case for \getenv
at the time, but upcoming regression test refactoring provides a
sufficient reason to add it now.

Discussion: https://postgr.es/m/1655733.1639871614@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/33d3eeadb21d2268104840cfef6bc2226ddfc680

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 22 ++++++++++++++++++++
src/bin/psql/command.c | 41 ++++++++++++++++++++++++++++++++++++++
src/test/regress/expected/psql.out | 12 +++++++++++
src/test/regress/sql/psql.sql | 12 +++++++++++
4 files changed, 87 insertions(+)