Restrict all names that will be used as directories to 255 chars

[jhclark]

We want to detect names that will cause problems on the filesystem as early as possible.

[jhclark]

The current plan is to have a special explicitly-enabled mode in which directory names > 255 characters will be md5 hashed into something smaller:

global { ducttape_hash_long_dirs=true }

On startup, ducttape will check if any directory names will violate this. If they do and the user has not enabled the hashing option, ducttape will exit with an error and tell them how to enable this option and why they must enable it.

To further make this less painful, ducttape will provide bash autocompletion for hashed names and will also provide a "lsd" option, which will ls realization directory names to stdout given a realization specifier, possibly containing globs.

[jhclark]

See d706884