Provide pg_preadv() and pg_pwritev().

Provide synchronous vectored file I/O routines.  These map to preadv()
and pwritev(), with fallback implementations for systems that don't have
them.  Also provide a wrapper pg_pwritev_with_retry() that automatically
retries on short writes.

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

configure.ac

@@ -1331,6 +1331,7 @@ AC_CHECK_HEADERS(m4_normalize([
 	sys/shm.h
 	sys/sockio.h
 	sys/tas.h
+	sys/uio.h
 	sys/un.h
 	termios.h
 	ucred.h
@@ -1660,9 +1661,14 @@ AC_CHECK_FUNCS(m4_normalize([
 	poll
 	posix_fallocate
 	ppoll
+	pread
+	preadv
 	pstat
 	pthread_is_threaded_np
+	pwrite
+	pwritev
 	readlink
+	readv
 	setproctitle
 	setproctitle_fast
 	setsid
@@ -1673,6 +1679,7 @@ AC_CHECK_FUNCS(m4_normalize([
 	sync_file_range
 	uselocale
 	wcstombs_l
+	writev
 ]))
 
 # These typically are compiler builtins, for which AC_CHECK_FUNCS fails.
@@ -1733,8 +1740,6 @@ AC_REPLACE_FUNCS(m4_normalize([
 	inet_aton
 	link
 	mkdtemp
-	pread
-	pwrite
 	random
 	srandom
 	strlcat