Modified remote file name syntax

[Michael Albinus <michael.albinus@gmx.de>]

Hi,

today I've committed a patch, which changes the remote file name syntax
in Emacs. The "method" part, optional until now, becomes
mandatory. Valid remote file names are now

/method:host:/path/to/file
/method:user@host:/path/to/file

A new method, "-", has been added which is a marker for the default
method as known so far.

The reason is, that all the time there have been problems in Tramp due
to syntactic ambiguities. A remote file name "/foo:" has not been
distinct; "foo" could be either a method or a host name. This has
resulted in permanent problems during file name completion.

The changes will be visible with the next releases of Tramp 2.3.2 and
Emacs 26.1.

Best regards, Michael.