pgsql: Introduce pg_pwrite_zeros() in fileutils.c

Introduce pg_pwrite_zeros() in fileutils.c

This routine is designed to write zeros to a file using vectored I/O,
for a size given by its caller, being useful when it comes to
initializing a file with a final size already known.

XLogFileInitInternal() in xlog.c is changed to use this new routine when
initializing WAL segments with zeros (wal_init_zero enabled). Note that
the aligned buffers used for the vectored I/O writes have a size of
XLOG_BLCKSZ, and not BLCKSZ anymore, as pg_pwrite_zeros() relies on
PGAlignedBlock while xlog.c originally used PGAlignedXLogBlock.

This routine will be used in a follow-up patch to do the pre-padding of
WAL segments for pg_receivewal and pg_basebackup when these are not
compressed.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, Michael
Paquier
Discussion: https://www.postgresql.org/message-id/CALj2ACUq7nAb7%3DbJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3bdbdf5d06f2179d4c17926d77ff734ea9e7d525

Modified Files
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src/backend/access/transam/xlog.c | 33 +++---------------
src/common/file_utils.c | 73 +++++++++++++++++++++++++++++++++++++++
src/include/common/file_utils.h | 2 ++
3 files changed, 80 insertions(+), 28 deletions(-)