nfsd: Add a new rc variable nfs_server_maxio

Since vfs.nfsd.srvmaxio can only be set when nfsd.ko
is loaded, but nfsd is not running, setting it in
/etc/sysctl.conf is not feasible when "options NFSD"
was not specified for the kernel.

This patch adds a new rc variable nfs_server_maxio,
which sets vfs.nfsd.srvmaxio at the correct time.

rc.conf.5 will be patched separately.

(cherry picked from commit 5509bad74e44eed3b9d54fbd74d3752b88428147)