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/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-02-15"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-02-17"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */