Use pg_pwrite_zeros() in walmethods.c

Use pg_pwrite_zeros() in walmethods.c

This change impacts pg_receivewal and pg_basebackup, for the pre-padding
with zeros of all the new non-compressed WAL segments, so as the code is
more robust on partial writes.  This makes the code consistent with the
backend (XLogFileInitInternal) when wal_init_zeros is enabled for the
WAL segment initialization.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, Michael
Paquier
Discussion: https://postgr.es/m/CALj2ACUq7nAb7=bJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA@mail.gmail.com