Terminology in multi-tty primitives

[Eli Zaretskii]

The terminology used by the multi-tty primitives is in its current
state very irregular, to the degree that IMO it hampers understanding
and promotes confusion.  Examples:

 . Sometimes we use "tty", as in `suspend-tty' and
   `make-frame-on-tty', and sometimes "terminal", as in
   `delete-terminal'.

 . `terminal-name' returns the name of the _terminal_device_, such as
   "/dev/tty", while a terminal object itself does not really have a
   name.

 . `get-device-terminal' accepts not only a device name (like
   "/dev/tty" or "foo:0.0"), as its name might suggest, but also a
   frame or a terminal.

 . Doc strings of several functions use the term "terminal id", but
   the functions accept a _terminal_object_, not an ID.  Since a
   terminal has an integer ID associated with it
   (cf. `get-device-terminal's return value), a user could easily be
   confused to think that we mean that integer identifier.

Are we OK with these inconsistencies?  If it's just me, I'm willing to
leave this alone and just document the status quo.  Otherwise, this is
our last chance to fix this mess before it is codified and frozen
forever.