Fix zpool subcommands error message with some unsupported options

[loli10K]

Motivation and Context

Both 'detach' and 'online' zpool subcommands, when provided with an unsupported option, forget to print it in the error message:

root@linux:/usr/src/zfs# ./cmd/zpool/zpool online -t rpool vda3
invalid option ''
usage:
	online [-e] <pool> <device> ...
root@linux:/usr/src/zfs#

Description

This changes fixes the error message in order to include the actual option that is not supported.

How Has This Been Tested?

Tested on local Debian10 builder, without patch:

root@linux:/usr/src/zfs# ./cmd/zpool/zpool online -t rpool vda3
invalid option ''
usage:
	online [-e] <pool> <device> ...

with patch applied:

root@linux:/usr/src/zfs# zpool online -t rpool vda3
invalid option 't'
usage:
	online [-e] <pool> <device> ...

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 #9270 into master will decrease coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9270      +/-   ##
==========================================
- Coverage   79.31%   79.07%   -0.25%     
==========================================
  Files         400      400              
  Lines      122034   122035       +1     
==========================================
- Hits        96792    96497     -295     
- Misses      25242    25538     +296

Flag	Coverage Δ
#kernel	79.75% <ø> (-0.07%)	⬇️
#user	66.55% <100%> (-0.91%)	⬇️

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[ghost]

Looks good. Those unused flags appear to have been vestigial from before the initial import for both ZoL and FreeBSD.