Use vectored I/O to fill new WAL segments.

Use vectored I/O to fill new WAL segments.

Instead of making many block-sized write() calls to fill a new WAL file
with zeroes, make a smaller number of pwritev() calls (or various
emulations).  The actual number depends on the OS's IOV_MAX, which
PG_IOV_MAX currently caps at 32.  That means we'll write 256kB per call
on typical systems.  We may want to tune the number later with more
experience.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJA%2Bu-220VONeoREBXJ9P3S94Y7J%2BkqCnTYmahvZJwM%3Dg%40mail.gmail.com