zpool: allow relative vdev paths

[tonyhutter]

Motivation and Context

Allow relative paths in zpool create.

Description

zpool create won't let you use relative paths to disks. This is annoying when you want to do:

zpool create tank ./diskfile

But have to do..

zpool create tank `pwd`/diskfile

This fixes it.

How Has This Been Tested?

Manually tested

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.