doc_open, doc_save and the other automation file paths only took
forward-slash relative paths, so an agent passing C:/Users/me/a.png with
a C:/ root was refused. An absolute path is now compared with the
root's absolute path as text: if it lies beneath the root, the root's
components are stripped and the rest goes through the same checks as a
relative path and is opened through the same directory capability.
Nothing is resolved on the filesystem, so traversal, prefixes, device
names and link escapes are refused as before; absolute paths outside
the root, or with no root granted, are still refused.
Fixes#2176.