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

src/test/regress/sql/psql.sql

@@ -141,6 +141,18 @@ select 'drop table gexec_test', 'select ''2000-01-01''::date as party_over'
 
 \unset FETCH_COUNT
 
+-- \setenv, \getenv
+
+-- ensure MYVAR isn't set
+\setenv MYVAR
+-- in which case, reading it doesn't change the target
+\getenv res MYVAR
+\echo :res
+-- now set it
+\setenv MYVAR 'environment value'
+\getenv res MYVAR
+\echo :res
+
 -- show all pset options
 \pset