Fix bugs with backslash escaping in interactive vi mode (#57)

This commit fixes the following bugs in the 'vi' editing mode
backslash escape feature. Ref.: Bolsky & Korn (1995), p. 113, which
states for \: "Similar to Control+V [...] except that it escapes
only the next Erase or Kill charactrer".

1. The vi mode now only escapes the next character if the last
   character input was a backslash, fixing the bug demonstrated at:
   https://asciinema.org/a/E3Rq3et07MMQG5BaF7vkXQTg0
2. Escaping backslashes are now disabled in vi.c if the vi mode is
   disabled (note that vi.c handles raw editing mode in UTF-8
   locales). This makes the behavior of the raw editing mode
   consistent in C/POSIX and UTF-8 locales.
3. An odd interaction with Backspace when the character prior to a
   separate buffer entered with Shift-C was a backslash has been
   fixed. Demonstration at: https://asciinema.org/a/314833
   ^? will no longer be output repeatedly when attempting to erase
   a separate buffer with a Backspace, although, to be consistent
   with vi(1), you still cannot backspace past it before escaping
   out of it. Ref.:
   https://github.com/ksh93/ksh/issues/56#issuecomment-653586994

src/cmd/ksh93/edit/vi.c:
- Prevent a backslash from escaping the next input if the previous
  input wasn't a backslash. This is done by unsetting a variable
  named backslash if a backslash escaped a character. backslash is
  set to the result of c == '\\' when the user enters a new
  character.
- Disable escaping backslashes in the raw editing mode because
  it should not be enabled there.

src/cmd/ksh93/tests/pty.sh:
- Add some tests for how ksh handles backslashes in each
  editing mode to test for the bugs fixed by this commit.

Fixes #56.

NEWS

@@ -3,6 +3,24 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-07-03:
+
+- Backslashes are no longer escaped in the raw Bourne Shell-like editing
+  mode in multibyte locales, i.e. backslashes are no longer treated like
+  Control-V if the emacs and vi modes are disabled.
+
+- Deleting a backslash in vi mode with Control-H or Backspace now only
+  escapes a backslash if it was the previous input. This means erasing a
+  string such as 'ab\\\' will only cause the first backslash to escape a
+  Backspace as '^?', like in emacs mode.
+
+- An odd interaction with Backspace when the last character of a separate
+  buffer created with Shift-C was '\' has been fixed. '^?' will no longer
+  be output repeatedly when attempting to erase a separate buffer with
+  a Backspace. Note that buffers created with Shift-C are not meant to be
+  erasable:
+  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/vi.html#tag_20_152_13_49
+
 2020-07-02:
 
 - Fixed a crash that occurred if a directory named '.paths' existed in any