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/sh.1

@@ -4969,7 +4969,7 @@ If the
 shell option is on (which is the default setting), this escapes the next character.
 Editing characters, the user's erase, kill and
 interrupt (normally
-.BR ^? )
+.BR ^C )
 characters
 may be entered
 in a command line or in a search string if preceded by a