Add 'x-ctrl-keysym' support on X window system

* src/xterm.c (x_x_to_emacs_modifiers, x_emacs_to_x_modifiers):
Support 'x-ctrl-keysym'.
(syms_of_xterm): DEFSYM "ctrl" and put a proper modifier-value
property on it.
<x-ctrl-keysym>: New DEFVAR_LISP.
<x-alt-keysym, x-hyper-keysym, x-meta-keysym, x-super-keysym>: Doc
fix.  (Bug#24822)

* etc/NEWS: Mention the addition of 'x-ctrl-keysym'.

* doc/lispref/os.texi (X11 Keysyms): Document 'x-ctrl-keysym'.

1 files changed, 6 insertions, 4 deletions

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 277abb1477d..97b086c0d37 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -2254,14 +2254,16 @@ The variable is always local to the current terminal, and cannot be
 buffer-local.  @xref{Multiple Terminals}.
 @end defvar
 
-You can specify which keysyms Emacs should use for the Meta, Alt, Hyper, and Super modifiers by setting these variables:
+You can specify which keysyms Emacs should use for the Control, Meta,
+Alt, Hyper, and Super modifiers by setting these variables:
 
-@defvar x-alt-keysym
+@defvar x-ctrl-keysym
+@defvarx x-alt-keysym
 @defvarx x-meta-keysym
 @defvarx x-hyper-keysym
 @defvarx x-super-keysym
-The name of the keysym that should stand for the Alt modifier
-(respectively, for Meta, Hyper, and Super).  For example, here is
+The name of the keysym that should stand for the Control modifier
+(respectively, for Alt, Meta, Hyper, and Super).  For example, here is
 how to swap the Meta and Alt modifiers within Emacs:
 @lisp
 (setq x-alt-keysym 'meta)