tcsh delete moves mark bug fix

<a href="http://www.tcsh.org/">
  <img class="right" src="tcsh_oystercatcher.jpg" /></a>

_This bug is fixed in tcsh versions 6.14.05 and later._

For the impatient:  
[Patch](tcsh-preserve-mark.patch) for
[tcsh 6.14.00](http://www.tcsh.org/MostRecentRelease)

I use [tcsh](http://www.tcsh.org/) as my shell, and I love it. Unfortunately,
version 6.14.00 introduced a bug in _backward-delete-word, backward-kill-line,
yank-pop_, and all other editor commands that use c_delbefore() in ed.chared.c.

If the mark is set in the middle of the region that's deleted, it's moved to
before the start of the deleted region. Specifically, it's moved backward by the
number of characters that were deleted.

To reproduce:

  1. Enter _foo barbaz_ at the prompt.

  2. Move the cursor to the _b_ in _baz_.

  3. Press Ctrl-Space to set the mark.

  4. Move the cursor to the end of the line.

  5. Press M-^H (Alt-Backspace) to delete _barbaz_.

The mark will now be on the first _o_ in _foo_. You can confirm by pressing ^W
to kill the region between the cursor and the mark. It will delete the _oo_,
leaving only _f_.

[This patch](tcsh-preserve-mark.patch)
against [tcsh 6.14.00](http://www.tcsh.org/MostRecentRelease) fixes the bug.