Update zfs program command usage

[behlendorf]

Motivation and Context

Issue #9056. Rather than try and force a particular non-portable
behavior for getopt(3) processing update the documentation to
require the -- option parsing terminator.

Description

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 ...

How Has This Been Tested?

The contrib/zcp/autosnap.lua script works as intended on Linux when
the -- is specified. Untested on other platforms, but their respective
getopt(3) man pages describe this functionality as supported.

This is only an update to the documentation.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9428 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9428      +/-   ##
==========================================
+ Coverage   79.02%   79.35%   +0.32%     
==========================================
  Files         410      410              
  Lines      122499   122499              
==========================================
+ Hits        96806    97210     +404     
+ Misses      25693    25289     -404

Flag	Coverage Δ
#kernel	79.83% <ø> (+0.07%)	⬆️
#user	67.29% <ø> (+0.51%)	⬆️

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[loli10K]

Tested on Illumos, looks good with and without --:

loli@openindiana:~$ uname -a
SunOS openindiana 5.11 master-0-g77ba330702 i86pc i386 i86pc
loli@openindiana:~$ wget -q https://raw.githubusercontent.com/zfsonlinux/zfs/master/contrib/zcp/autosnap.lua -O autosnap.lua
loli@openindiana:~$ sudo truncate -s 1g /var/tmp/file-vdev
loli@openindiana:~$ sudo zpool create -f -m none testpool /var/tmp/file-vdev
loli@openindiana:~$ sudo zfs create testpool/fs1
loli@openindiana:~$ sudo zfs create testpool/fs2
loli@openindiana:~$ sudo zfs set org.zfsonlinux:auto-snapshot=true testpool
loli@openindiana:~$ sudo zfs set org.illumos:auto-snapshot=true testpool
loli@openindiana:~$ 
loli@openindiana:~$ sudo zfs program testpool autosnap.lua -- -p org.zfsonlinux:auto-snapshot testpool@snap1
Channel program fully executed and produced output:
    return:
        testpool/fs2@snap1: 0
        testpool@snap1: 0
        testpool/fs1@snap1: 0
loli@openindiana:~$ sudo zfs program testpool autosnap.lua -p org.illumos:auto-snapshot testpool@snap2
Channel program fully executed and produced output:
    return:
        testpool/fs1@snap2: 0
        testpool/fs2@snap2: 0
        testpool@snap2: 0
loli@openindiana:~$ zfs list -t snap -r testpool
NAME                 USED  AVAIL  REFER  MOUNTPOINT
testpool@snap1          0      -    24K  -
testpool@snap2          0      -    24K  -
testpool/fs1@snap1      0      -    24K  -
testpool/fs1@snap2      0      -    24K  -
testpool/fs2@snap1      0      -    24K  -
testpool/fs2@snap2      0      -    24K  -
loli@openindiana:~$