Fix emacs backslash escaping behavior (#179)

This commit fixes the following:

1. Emacs mode ignores --nobackslashctrl (re: 24598fed) when in
   reverse search.

2. When entering more than one backslash, emacs reverse search mode
   deletes multiple backslashes after pressing backspace once.
   Reproducer:
   $ set --emacs --nobackslashctrl
   $ <Ctrl+R> \\\\<Backspace>

3. Except when in reverse search, the backslash fails to escape a
   subsequent interrupt character (^C). Reproducer:
   $ set --emacs --backslashctrl
   $ teststring \<Ctrl+C>

src/cmd/ksh93/edit/emacs.c:
- Disable escaping backslashes in emacs reverse search if
  'nobackslashctrl' is enabled.
- Fix the buggy behavior of backslashes in emacs reverse
  search by processing backslashes in a loop.

src/cmd/ksh93/tests/pty.sh:
- Add regression tests.

src/cmd/ksh93/sh.1:
- Fix a minor documentation error (^C is the usual interrupt
  character, not ^?).

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/tests/pty.sh

@@ -640,5 +640,35 @@ r ^/dev/null\r\n$
 r ^:test-2:
 !
 
+# err_exit #
+((SHOPT_ESH)) && [[ -o ?backslashctrl ]] && tst $LINENO <<"!"
+L nobackslashctrl in emacs
+
+d 10
+w set -o emacs --nobackslashctrl
+
+# --nobackslashctrl shouldn't be ignored by reverse search
+p :test-2:
+w \cR\\\cH\cH
+r ^:test-2: \r\n$
+!
+
+# err_exit #
+((SHOPT_ESH)) && tst $LINENO <<"!"
+L emacs backslash escaping
+
+d 10
+w set -o emacs
+
+# Test for too many backslash deletions in reverse-search mode
+p :test-2:
+w \cRset\\\\\\\\\cH\cH\cH\cH\cH
+r ^:test-2: set -o emacs$
+
+# \ should escape the interrupt character (usually Ctrl+C)
+w true \\\cC
+r true \^C
+!
+
 # ======
 exit $((Errors<125?Errors:125))