Correct zfs-send(8) on readonly sends

[rincebrain]

Motivation and Context

zfs-send(8) claimed you could use -pR when sending a readonly filesystem
or volume. You cannot.

The usage output for zfs send already reflects this.

Description

As above.

I suspect this never worked, but admit I have not tried firing up an old version or going source spelunking to verify this - I just got burned by this when helping someone send|recv from a readonly import during recovery, and wanted to at a minimum ensure nobody else is misled by the man page.

(I also noticed testing for other discrepancies between the man page and usage output for the r/o case that the usage didn't list -D even though it "works" fine. I know it's a noop, and going away Real Soon Now(tm), but it seemed better to list it (and eventually have it removed with the other instances) than either leaving the discrepancy or deleting it from the man page.)

How Has This Been Tested?

Well, I ran mancheck, and looked at the man output.

If I broke something with the zfs_main.c edit, I will be quite impressed.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.