Update zfs program command usage

Update the zfs(8) man page to clearly describe that arguments for
channel programs are to be listed after the -- sentinel which
terminates argument processing.  This behavior is supported by
getopt on Linux, FreeBSD, and Illumos according to each platforms
respective man pages.

    zfs program [-jn] [-t instruction-limit] [-m memory-limit]
        pool script [--] arg1 ...

Reviewed-by: Clint Armstrong <clint@clintarmstrong.net>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9056 
Closes #9428

man/man8/zfs.8

@@ -316,7 +316,8 @@
 .Op Fl t Ar instruction-limit
 .Op Fl m Ar memory-limit
 .Ar pool script
-.Op Ar arg1 No ...
+.Op --
+.Ar arg1 No ...
 .Nm
 .Cm load-key
 .Op Fl nr
@@ -4756,7 +4757,8 @@ Display the path's inode change time as the first column of output.
 .Op Fl t Ar instruction-limit
 .Op Fl m Ar memory-limit
 .Ar pool script
-.Op Ar arg1 No ...
+.Op --
+.Ar arg1 No ...
 .Xc
 Executes
 .Ar script