libnfs.a: move mnt_{open, close}clnt calls to utils/mount/network.c

It turns out that get_socket() accesses a global variable, "verbose," that
is only available in the mount command; yet it's in libnfs.a.  This creates
an undocumented API dependency that will bite someone someday.  This
mount-specific functionality doesn't really belong in libnfs.a anyway.

The simplest way to resolve this is to move all of the functions in
support/nfs/conn.c into utils/mount.  network.c seems like the logical
place to put these.  An added benefit is we eventually get to make
get_socket() static.

Let's start with the mnt_{open,close}clnt functions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>