Allow to store string values for file-tag relationship explanations

[mwatts15]

Earlier, it was intended that there be values for a file-tag relationship, but later, this feature was walked back since most of the cases where values might be wanted, it's more appropriate to put the value in the tag in a scheme like "tag:value" for categorical values, or "tag:value-range-designator" for continuous values, where "value-range-designator" is something like "low", "medium", "high", or "0-1023", "1024-1048575", etc. An alternative use for having values associated to tag is to leave an explanation as to why a given file has the tag. This is especially useful once you want to share files with their tagging metadata.

Subtasks

• Add a command to create a tag and set the default explanation.
• Fully implement getxattr -- currently always returns an empty value, even for extended attributes on the underlying file.
• Possibly update the database tables so tag(default_value) becomes tag(default_explanation) and file_tag(value) becomes file_tag(explanation)
• Fully implement setxattr -- values aren't set for any tag
• Possibly allow to store non-null terminated values -- it may be useful in some case.