input: Do not set high-bit; preserve ALT modifier.

Background: Vim internally prefers to represent ALT/META chords as
single-byte keys, by setting the high bit of the key byte.
extract_modifiers() _discards_ the meta/alt modifier, but we need it for
libvterm and libtermkey.

Closes #2440
Closes #3727
Closes #2017
References #2277
References #2254

https://github.com/neovim/neovim/issues/2017#issuecomment-140423557
> We [not libtermkey] are setting the high bit for some reason

https://github.com/neovim/neovim/issues/176#issuecomment-77834715
> libvtermkey requires the leading esc to parse alt/meta

https://github.com/neovim/neovim/pull/3246#issuecomment-136328450
> A program could do better than the current logic on some terminals, by
> asking for pure 8bit mode (S8C1T) and then immediately querying the
> mode again. If the result comes back as an 8bit single-byte CSI, then
> it can presume the mode setting was successful, and now the ESC prefix
> byte won't be seen in multibyte sequences; only as an Alt- prefix or
> a real Escape key. On such a terminal, it could therefore avoid
> needing to use that waiting timeout.

src/nvim/terminal.c

@@ -1,5 +1,5 @@
-// VT220/xterm-like terminal emulator implementation for Neovim. Powered by
-// libvterm(http://www.leonerd.org.uk/code/libvterm/).
+// VT220/xterm-like terminal emulator implementation for nvim. Powered by
+// libvterm (http://www.leonerd.org.uk/code/libvterm/).
 //
 // libvterm is a pure C99 terminal emulation library with abstract input and
 // display. This means that the library needs to read data from the master fd
@@ -10,31 +10,31 @@
 // vterm_keyboard_key/vterm_keyboard_unichar, which generates byte streams that
 // must be fed back to the master fd.
 //
-// This implementation uses Neovim buffers as the display mechanism for both
+// This implementation uses nvim buffers as the display mechanism for both
 // the visible screen and the scrollback buffer. When focused, the window
 // "pins" to the bottom of the buffer and mirrors libvterm screen state.
 //
 // When a line becomes invisible due to a decrease in screen height or because
 // a line was pushed up during normal terminal output, we store the line
-// information in the scrollback buffer, which is mirrored in the Neovim buffer
+// information in the scrollback buffer, which is mirrored in the nvim buffer
 // by appending lines just above the visible part of the buffer.
 //
 // When the screen height increases, libvterm will ask for a row in the
-// scrollback buffer, which is mirrored in the Neovim buffer displaying lines
+// scrollback buffer, which is mirrored in the nvim buffer displaying lines
 // that were previously invisible.
 //
-// The vterm->Neovim synchronization is performed in intervals of 10
+// The vterm->nvim synchronization is performed in intervals of 10
 // milliseconds. This is done to minimize screen updates when receiving large
 // bursts of data.
 //
 // This module is decoupled from the processes that normally feed it data, so
-// it's possible to use it as a general purpose console buffer(possibly as a
-// log/display mechanism for Neovim in the future)
+// it's possible to use it as a general purpose console buffer (possibly as a
+// log/display mechanism for nvim in the future)
 //
-// Inspired by vimshell(http://www.wana.at/vimshell/) and
-// Conque(https://code.google.com/p/conque/).  Libvterm usage instructions (plus
+// Inspired by vimshell (http://www.wana.at/vimshell/) and
+// Conque (https://code.google.com/p/conque/). Libvterm usage instructions (plus
 // some extra code) were taken from
-// pangoterm(http://www.leonerd.org.uk/code/pangoterm/)
+// pangoterm (http://www.leonerd.org.uk/code/pangoterm/)
 #include <assert.h>
 #include <stdio.h>
 #include <stdint.h>
@@ -402,11 +402,11 @@ static int terminal_execute(VimState *state, int key)
   TerminalState *s = (TerminalState *)state;
 
   switch (key) {
-    case K_FOCUSGAINED:  // Neovim has been given focus
+    case K_FOCUSGAINED:  // nvim has been given focus
       apply_autocmds(EVENT_FOCUSGAINED, NULL, NULL, false, curbuf);
       break;
 
-    case K_FOCUSLOST:   // Neovim has lost focus
+    case K_FOCUSLOST:   // nvim has lost focus
       apply_autocmds(EVENT_FOCUSLOST, NULL, NULL, false, curbuf);
       break;