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

src/interfaces/libpq/fe-print.c

@@ -165,6 +165,13 @@ PQprint(FILE *fout, const PGresult *res, const PQprintOpt *po)
 			screen_size.ws_row = 24;
 			screen_size.ws_col = 80;
 #endif
+
+			/*
+			 * Since this function is no longer used by psql, we don't examine
+			 * PSQL_PAGER.  It's possible that the hypothetical external users
+			 * of the function would like that to happen, but in the name of
+			 * backwards compatibility, we'll stick to just examining PAGER.
+			 */
 			pagerenv = getenv("PAGER");
 			/* if PAGER is unset, empty or all-white-space, don't use pager */
 			if (pagerenv != NULL &&