Fix pg_rewind when pg_xlog is a symlink.

pg_xlog is often a symlink, typically to a different filesystem. Don't
get confused and comlain about by that, and just always pretend that it's a
normal directory, even if it's really a symlink.

Also add a test case for this.

Backpatch to 9.5.

src/bin/pg_rewind/t/002_databases.pl

@@ -10,6 +10,7 @@ sub run_test
 	my $test_mode = shift;
 
 	RewindTest::setup_cluster();
+	RewindTest::start_master();
 
 	# Create a database in master.
 	master_psql('CREATE DATABASE inmaster');