small changes for plan 9 in Truncate and GetFileInfo; NFS server now works on Plan 9

[rminnich]

No description provided.

[willscott]

I don't understand either of these changes

• first commit resubmits Add plan 9 support for osFileInfo #141 which you haven't justified
• second commit does not apply to truncate, and I can't find any documentation for why the 'exclusive' bit in opening a file is an issue in plan 9

[rminnich]

ah, file_other.go was not there when I wrote file_plan9.go

As for O_EXCL, I see the actual problem:
linux:
" In general, the behavior of O_EXCL is undefined if it is used without O_CREAT. There is one ex‐
ception: on Linux 2.6 and later, O_EXCL can be used without O_CREAT if pathname refers to a block
device. If the block device is in use by the system (e.g., mounted), open() fails with the error
EBUSY."

Plan 9: O_EXCL only makes sense for create.

The flags currently are: os.O_WRONLY|os.O_EXCL

I don't think it even makes sense to use O_EXCL at all here, but on Plan 9 you get an error, and on linux you don't.

I think just dropping O_EXCL completely may be the proper fix. But the combination of os.O_WRONLY|os.O_EXCL gets an error on Plan 9.