os: add ModeIrregular flag

[dsnet]

The os.FileInfo type is an interface, allowing other packages to implement it. One of the fields it returns is Mode which reports two pieces of information: 1) what the permissions are, and 2) what type of file it is. There is a helper method os.FileMode.IsRegular that reports whether the file is regular or not. Currently the only way to have IsRegular report false is to set the type to be one of the following:

• ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice

For some implementations of os.FileInfo (see #22903 for an example), we want to be able to indicate that the file is not regular. However, every one of the aforementioned types to indicate non-regular are ill-suited as a general-purpose "unknown" file type.

Thus, I propose adding a ModeUnknown to serve exactly for that purpose. The ModeUnknown type carries no information other than the fact that the file is not regular.

CL/94856 is one such implementation.

\cc @bradfitz @ianlancetaylor @robpike @rsc

[rsc]

"Unknown" sounds like not knowing anything. We know it's not regular.
ModeOther? ModeIrregular? ModeSpecial?

[rsc]

Does sound worth doing though, modulo the name.

[dsnet]

ModeIrregular SGTM. The name exactly reflects what it means.

[gopherbot]

Change https://golang.org/cl/94856 mentions this issue: os: add ModeIrregular flag

[bradfitz]

Was reverted. Reopening.

[dsnet]

We ended up rolling forward with a fix in: https://go-review.googlesource.com/103616
So the revert didn't happen.

[bradfitz]

Ah, whoops. Great. I approved the API change (+ saw the revert CL created) but I missed that the revert was abandoned.