Tramp and VC integration: "calling user"

[kai@emptydomain.de (Kai Großjohann)]

Tramp advises some VC functions.  It is bad for one package in Emacs
to advise functions in another.

In one of those cases, VC wants to find out if a file is locked by the
calling user.  To do this, VC invokes a command to print the name of
the user who locked the file, and then compares that name with
user-login-name.

For remote files, the comparison should not be with user-login-name,
it should instead be with the user logged into the remote host in
question.

Suggestions for solving this:

(1) New file operation file-mine-p, returns true if the file is owned
    by the "calling user".  For non-special files, the calling user is
    the user who invoked Emacs.  For Tramp files, the calling user is
    the user logged into the remote host.

(2) New file operation file-calling-user, returns the calling user, as
    defined in (1).

(3) Augment the return value of file-remote-p to indicate the calling
    user.  The return value could be augmented to also indicate the
    remote host, if the file is remote.

What do people think?

Kai