Make patch_volume prefer rw over ro

[andyneff]

#164 introduced a behavior to prefer the shortest remainder. This would cover

SOURCE_DIR:/src:ro
SOURCE_DIR/some_sub_dir:/processing_dir:rw

While this works with this situation, the ro/rw flag could be swapped, and we would be back to potentially getting :ro when we wanted :rw. As this is a case of intention that cannot be disambiguated, the most robust method would be to sacrifice :ro protection for :rw versatility.

• Add ro/rw flags to volume map
• Prefer rw over ro, but still choose the shortest remainder
• Add a warning that this is happening. I think the warning message should only happen when the shortest remainder isn't already rw, or else this will trigger false positives 100% of the time.