libstand: increase nfs max read size to 16k

With ip fragment reassembly implemented, it makes sense to allow
larger nfs reads. Note due to loader heap size limit, we do not want
to set too large maximum read size. Also we do not change default read size.

Reviewed by:	bcr, allanjude
Differential Revision:	https://reviews.freebsd.org/D10754

lib/libstand/nfs.c

@@ -52,7 +52,7 @@ __FBSDID("$FreeBSD$");
 #define NFS_DEBUGxx
 
 #define NFSREAD_MIN_SIZE 1024
-#define NFSREAD_MAX_SIZE 4096
+#define NFSREAD_MAX_SIZE 16384
 
 /* NFSv3 definitions */
 #define	NFS_V3MAXFHSIZE		64

sys/boot/i386/pxeldr/pxeboot.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 1, 2000
+.Dd May 16, 2017
 .Dt PXEBOOT 8
 .Os
 .Sh NAME
@@ -91,7 +91,7 @@ This may be changed by setting the
 .Va nfs.read_size
 variable in
 .Pa /boot/loader.conf .
-Valid values range from 1024 to 4096 bytes.
+Valid values range from 1024 to 16384 bytes.
 .Pp
 In all other respects,
 .Nm