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 413ccaa74d), 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

doc/src/sgml/ref/pg_restore.sgml

@@ -176,8 +176,8 @@
       <listitem>
        <para>
         Specify output file for generated script, or for the listing
-        when used with <option>-l</option>. Default is the standard
-        output.
+        when used with <option>-l</option>. Use <literal>-</literal>
+        for the standard output, which is also the default.
        </para>
       </listitem>
      </varlistentry>

src/bin/pg_dump/pg_backup_archiver.c

@@ -1511,7 +1511,12 @@ SetOutput(ArchiveHandle *AH, const char *filename, int compression)
 	int			fn;
 
 	if (filename)
-		fn = -1;
+	{
+		if (strcmp(filename, "-") == 0)
+			fn = fileno(stdout);
+		else
+			fn = -1;
+	}
 	else if (AH->FH)
 		fn = fileno(AH->FH);
 	else if (AH->fSpec)

src/bin/pg_dump/pg_restore.c

@@ -454,7 +454,7 @@ usage(const char *progname)
 
 	printf(_("\nGeneral options:\n"));
 	printf(_("  -d, --dbname=NAME        connect to database name\n"));
-	printf(_("  -f, --file=FILENAME      output file name\n"));
+	printf(_("  -f, --file=FILENAME      output file name (- for stdout)\n"));
 	printf(_("  -F, --format=c|d|t       backup file format (should be automatic)\n"));
 	printf(_("  -l, --list               print summarized TOC of the archive\n"));
 	printf(_("  -v, --verbose            verbose mode\n"));