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/edit/edit.c

@@ -1068,7 +1068,7 @@ int ed_getchar(register Edit_t *ep,int mode)
 	{
 		ed_flush(ep);
 		ep->e_inmacro = 0;
-		/* The while is necessary for reads of partial multbyte chars */
+		/* The while is necessary for reads of partial multibyte chars */
 		*ep->e_vi_insert = (mode==-2);
 		if((n=ed_read(ep,ep->e_fd,readin,-LOOKAHEAD,0)) > 0)
 			n = putstack(ep,readin,n,1);