nfscl: Add a Linux compatible "nconnect" mount option

Linux has had an "nconnect" NFS mount option for some time.
It specifies that N (up to 16) TCP connections are to created for a mount,
instead of just one TCP connection.

A discussion on freebsd-net@ indicated that this could improve
client<-->server network bandwidth, if either the client or server
have one of the following:

• multiple network ports aggregated to-gether with lagg/lacp.
• a fast NIC that is using multiple queues

It does result in using more IP port#s and might increase server
peak load for a client.

One difference from the Linux implementation is that this implementation
uses the first TCP connection for all RPCs composed of small messages
and uses the additional TCP connections for RPCs that normally have
large messages (Read/Readdir/Write). The Linux implementation spreads
all RPCs across all TCP connections in a round robin fashion, whereas
this implementation spreads Read/Readdir/Write across the additional
TCP connections in a round robin fashion.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30970