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.

src/cmd/ksh93/tests/pty.sh

@@ -505,5 +505,38 @@ r ^:test-2: true (/de\tv/nu\tl\tl|/de       v/nu    l       l)\r\n$
 p :test-3:
 !
 
+LC_ALL=C.UTF8 tst $LINENO <<"!"
+L raw Bourne mode backslash handling
+
+# The escaping backslash feature should be disabled in the raw Bourne mode.
+# This is tested with both erase and kill characters.
+
+d 10
+p :test-1:
+w set +o vi +o emacs; stty erase ^H kill ^X
+p :test-2:
+w true string\\\\\cH\cH
+r ^:test-2: true string\r\n$
+p :test-3:
+w true incorrect\\\cXtrue correct
+r ^:test-3: true correct\r\n$
+!
+
+for mode in emacs vi; do
+tst $LINENO << !
+L escaping backslashes in $mode mode
+
+# Backslashes should only be escaped if the previous input was a backslash.
+# Other backslashes stored in the input buffer should be erased normally.
+
+d 10
+p :test-1:
+w set -o $mode; stty erase ^H
+p :test-2:
+w true string\\\\\\\\\\cH\\cH\\cH
+r ^:test-2: true string\\r\\n$
+!
+done
+
 # ======
 exit $((Errors<125?Errors:125))