Change pg_restore -f- to dump to stdout instead of to ./-

Starting with PostgreSQL 12, pg_restore refuses to run when neither -d
nor -f are specified (c.f. commit 413ccaa7), and it also makes "-f -"
mean the old implicit behavior of dumping to stdout.  However, older
branches write to a file called ./- when invoked like that, making it
impossible to write pg_restore scripts that work across versions.  This
is a partial backpatch of the aforementioned commit to all older
supported branches, providing an upgrade path.

Discussion: https://postgr.es/m/20191006190839.GE18030@telsasoft.com