Improve command-line parser: "\word" forces label

This allows the same function to be used with different commands.
Recent improvements to the parser stopped adding labels for bare words
"word" in the CLI schema, to reduce the chance of label conflicts, eg {
"get", "file", "<file>" }.  Now a backslash-prepended word in the schema
"\word" will cause the matched argument to be labelled "word", just like
"[word]" does except "\word" is not optional.
More progress on Rhizome mirror script