Don't choke on files that are removed while pg_rewind runs.

If a file is removed from the source server, while pg_rewind is running, the
invocation of pg_read_binary_file() will fail. Use the just-added missing_ok
option to that function, to have it return NULL instead, and handle that
gracefully. And similarly for pg_ls_dir and pg_stat_file.

Reported by Fujii Masao, fix by Michael Paquier.