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

sys/compat/freebsd32/syscalls.master

@@ -1164,5 +1164,8 @@
 				    char *buf, size_t size, int flags); }
 575	AUE_CLOSERANGE	NOPROTO	{ int close_range(u_int lowfd, u_int highfd, \
 				    int flags); }
+; 576 is initialised by the krpc code, if present.
+576	AUE_NULL	NOSTD|NOPROTO	{ int rpctls_syscall(int op, \
+				    const char *path); }
 
 ; vim: syntax=off

sys/kern/syscalls.master

@@ -3234,6 +3234,13 @@
 		    int flags
 		);
 	}
+; 576 is initialised by the krpc code, if present.
+576	AUE_NULL	NOSTD {
+		int rpctls_syscall(
+		    int op,
+		    _In_z_ const char *path
+		);
+	}
 
 ; Please copy any additions and changes to the following compatability tables:
 ; sys/compat/freebsd32/syscalls.master