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