process-tty-name of processes on remote hosts

[Michael Albinus]

While working on tramp and gdb-mi for Bug#4604 and Bug#6360, there has
been a problem with `process-tty-name'. For processes started by
`start-file-process' this function returns the tty name of the local
host the process is bound to. This is not sufficient; sometimes the tty
name on the remote host must be known.

A solution by a file name handler is not possible; `process-tty-name'
does not know about magic file names.

One possiblity would be to introduce a new function
`set-process-tty-name', which could be called by Tramp, and which would
allow `process-tty-name' to return the proper value.

If it shall be still supported to distinguish the "local" and "remote"
tty names, a new function `process-remote-tty-name' could be introduced,
which DTRT.

Or Tramp simply sets a process property 'remote-tty, which could be
checked by other packages with

  (or (process-get proc 'remote-tty)
      (process-tty-name proc))

What do people think?

Best regards, Michael.