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.

Updates #22903
Fixes #23878

Change-Id: I4f34d88f960bcb014816d8e7b5de8b1035077948
Reviewed-on: https://go-review.googlesource.com/94856Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>