There is currently no way for os.FileMode.IsRegular to report false
without being one of the following types:
ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice
This makes it difficult for custom implementations of os.FileInfo to return
a Mode that is explicitly not regular without resorting to setting one
of the types listed above. However, every one of the aforementioned types
are ill-suited as a general-purpose "not regular" file type.
Thus, add a ModeIrregular to serve exactly for that purpose.
The ModeIrregular type carries no information other than the fact that the
file is not regular.