Default to wal_sync_method=fdatasync on FreeBSD.

FreeBSD 13 gained O_DSYNC, which would normally cause wal_sync_method to
choose open_datasync as its default value.  That may not be a good
choice for all systems, and performs worse than fdatasync in some
scenarios.  Let's preserve the existing default behavior for now.

Like commit 576477e73c, which did the same for Linux, back-patch to all
supported releases.

Discussion: https://postgr.es/m/CA%2BhUKGLsAMXBQrCxCXoW-JsUYmdOL8ALYvaX%3DCrHqWxm-nWbGA%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -2928,7 +2928,7 @@ include_dir 'conf.d'
         Not all of these choices are available on all platforms.
         The default is the first method in the above list that is supported
         by the platform, except that <literal>fdatasync</literal> is the default on
-        Linux.  The default is not necessarily ideal; it might be
+        Linux and FreeBSD.  The default is not necessarily ideal; it might be
         necessary to change this setting or other aspects of your system
         configuration in order to create a crash-safe configuration or
         achieve optimal performance.

src/backend/utils/misc/postgresql.conf.sample

@@ -206,7 +206,7 @@
 #wal_sync_method = fsync		# the default is the first option
 					# supported by the operating system:
 					#   open_datasync
-					#   fdatasync (default on Linux)
+					#   fdatasync (default on Linux and FreeBSD)
 					#   fsync
 					#   fsync_writethrough
 					#   open_sync

src/include/port/freebsd.h

@@ -1 +1,10 @@
 /* src/include/port/freebsd.h */
+
+/*
+ * Set the default wal_sync_method to fdatasync.  xlogdefs.h's normal rules
+ * would prefer open_datasync on FreeBSD 13+, but that is not a good choice on
+ * many systems.
+ */
+#ifdef HAVE_FDATASYNC
+#define PLATFORM_DEFAULT_SYNC_METHOD	SYNC_METHOD_FDATASYNC
+#endif