Revise and improve diskfile_is_name_unique()

Prior to this, the API would only check for a file's uniqueness by
looking up the bug_file table, and did not actually checking the file
system for an existing file.

This causes 2 potential issues:

• we could have a duplicate in the file system and therefore unwittingly
  overwrite it with a new file
• it was not possible to use the function for for project docs

Fixes 0015572