libstand: increase nfs max read size to 16k

[tsoome]

Summary

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.

[tsoome]

update pxeboot manual.

[bcr]

You need to bump the .Dd as you made a content change in the man page.

[tsoome]

.Dd update for manual.

[tsoome]

In D10754#222704, @bcr wrote:

You need to bump the .Dd as you made a content change in the man page.

Yea, forgot about it, thanks.

[bcr]

Approved from manpages. Thanks!