Add a syscall for the nfs-over-tls daemons to use.

The nfs-over-tls daemons need a system call to perform operations such as
associate a file descriptor with a krpc socket.
The daemons will not be in head for some time, but it will make it
easier for testers of nfs-over-tls to do testing if the system call
is in head (basically the stub for libc which will be commited soon).

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D24949