bug#23009: 25.0.92; xterm-mouse-mode should not assume UTF-8 coordinates

[Philipp Stephani]

`xterm-mouse-mode' sends \e[?1005h to enable UTF-8 mouse coordinates,
but it never checks that the terminal actually supports that.  Then
`xterm-mouse--read-utf8-char' reads an UTF-8 sequence without checking
that the terminal actually sent one.  On terminals that don't support
UTF-8 coordinates (such as Chromium's hterm) this leads to garbage if
any mouse coordinate is larger than 127.  For example, when clicking in
the right part of the terminal in hterm, the menu opens and the
following is shown in lossage:

   ESC [ M SPC \237 F ESC [ M # \237 F [tmm-menubar-mouse]

Instead, this should have been interpreted as two events with
single-byte mouse coordinates.

I'd suggest the following (and will happily provide patches if
accepted):

- Don't enable UTF-8 coordinates at all.  It is too hard to figure out
  whether they are enabled.  Rather, err on the safe side and only use
  single-byte coordinates.  (The superior SGR mode, which doesn't suffer
  from this problem, should remain enabled.)

- In `xterm-mouse--read-number-from-terminal', specify a very short
  timeout.  The terminal will always write the entire escape sequence as
  a unit, so waiting only increases the chance to accidentially read
  unrelated events.

- As an optimization, provide a `read-byte' function written in C that
  reads a single byte without taking the current terminal encoding into
  account.



In GNU Emacs 25.0.92.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2016-03-14 built on REDACTED
Repository revision: 80ec484ac83e6965a843dabf766ade057ee1bc6a
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04 LTS

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