os: add ModeIrregular flag

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>

src/os/types.go

@@ -54,15 +54,16 @@ const (
 	ModeSetgid                                     // g: setgid
 	ModeCharDevice                                 // c: Unix character device, when ModeDevice is set
 	ModeSticky                                     // t: sticky
+	ModeIrregular                                  // ?: non-regular file; nothing else is known about this file
 
 	// Mask for the type bits. For regular files, none will be set.
-	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice
+	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice | ModeIrregular
 
 	ModePerm FileMode = 0777 // Unix permission bits
 )
 
 func (m FileMode) String() string {
-	const str = "dalTLDpSugct"
+	const str = "dalTLDpSugct?"
 	var buf [32]byte // Mode is uint32.
 	w := 0
 	for i, c := range str {