Implement 1-based column numbering in mode line

* src/xdisp.c (decode_mode_spec): Implement the %C construct.

* lisp/bindings.el (column-number-indicator-zero-based): New
defcustom.
(mode-line-position): Use %C when
column-number-indicator-zero-based is nil.

* src/xdisp.c (syms_of_xdisp) <frame-title-format>:
* src/buffer.c (syms_of_buffer) <mode-line-format>:
* doc/lispref/modes.texi (%-Constructs):
* doc/lispref/frames.texi (Frame Titles): Document the %C
construct.

* doc/emacs/display.texi (Optional Mode Line): Document
'column-number-indicator-zero-based'.

* etc/NEWS: Mention 'column-number-indicator-zero-based' and the
%C construct.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 33ffdf8eb8e..80dbd3318dc 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5486,8 +5486,11 @@ A string is printed verbatim in the mode line except for %-constructs:
         For a modified read-only buffer, %* gives % and %+ gives *.
   %s -- print process status.   %l -- print the current line number.
   %c -- print the current column number (this makes editing slower).
+        Columns are numbered starting from the left margin, and the
+        leftmost column is displayed as zero.
         To make the column number update correctly in all cases,
         `column-number-mode' must be non-nil.
+  %C -- Like %c, but the leftmost column is displayed as one.
   %i -- print the size of the buffer.
   %I -- like %i, but use k, M, G, etc., to abbreviate.
   %p -- print percent of buffer above top of window, or Top, Bot or All.