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 <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Reviewed-by: loli10K <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #9056
Closes #9428

man/man8/zfs.8

@@ -305,7 +305,8 @@
 .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
@@ -4469,7 +4470,8 @@ 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