printf: Fix HTML and URI encoding (%H, %#H)

This applies a number of fixes to the printf formatting directives
%H and %#H (as well as their equivalents %(html)q and %(url)q):
1. Both formatters have been made multibyte/UTF-8 aware, and no
   longer delete multibyte characters. Invalid UTF-8 byte sequences
   are rendered as ASCII question marks.
2. %H no longer wrongly encodes spaces as non-breaking spaces
   ( ) and instead correctly encodes the UTF-8 non-breaking
   space as such.
3. %H now converts the single quote (') to '%#39;' instead of
   ''' which is not a valid entity in all HTML versions.
4. %#H failed to encode some reserved characters (e.g. '?') while
   encoding some unreserved ones (e.g. '~'). It now percent-encodes
   all characters except those 'unreserved' as per RFC3986 (ASCII
   alphanumeric plus -._~).

Prior discussion:
ce8d1467-4a6d-883b-45ad-fc3c7b90e681%40inlv.org

src/cmd/ksh93/include/defs.h:
src/cmd/ksh93/sh/string.c:
- defs.h: If compiling without SHOPT_MULTIBYTE, redefine the
  mbwide() macro (which tests if we're in a multibyte locale) as 0.
  This lets the compiler optimiser do the work that would otherwise
  require a lot of tedious '#if SHOPT_MULTIBYTE' directives.
- string.c: Remove some now-unneeded '#if SHOPT_MULTIBYTE' stuff.
- defs.h, string.c: Rename is_invisible() to sh_isprint(), invert
  the boolean return value, and make it an extern for use in
  fmthtml() -- see below. If compiling without SHOPT_MULTIBYTE,
  simply #define sh_isprint() as equivalent to isprint(3).
- defs.h: Add URI_RFC3986_UNRESERVED macro for fmthtml() containing
  the characters "unreserved" for purposes of URI percent-encoding.

src/cmd/ksh93/bltins/print.c: fmthtml():
- Remove kludge that skipped all multibyte characters (!).
- Complete rewrite to implement fixes described above.
- Don't bother with '#if SHOPT_MULTIBYTE' directives (see above).

src/cmd/ksh93/data/builtins.c:
- sh_optprintf[]: %H: Add single quote to encoded chars doc.
- Edit credits and bump version date.

src/cmd/ksh93/tests/builtins.sh:
- Update and tweak old regression tests.
- Add a number of new tests for UTF-8 HTML and URI encoding, which
  are only run when running tests in a UTF-8 locale (shtests -u).

src/cmd/ksh93/include/defs.h

@@ -29,6 +29,11 @@
 #define defs_h_defined
 
 #include	<ast.h>
+#if !SHOPT_MULTIBYTE
+#   undef mbwide
+#   define mbwide() (0)	/* disable multibyte without need for further '#if SHOPT_MULTIBTYE' */
+#endif
+
 #include	<sfio.h>
 #include	<error.h>
 #include	"FEATURE/externs"
@@ -441,6 +446,14 @@ extern int 		sh_whence(char**,int);
     extern Namval_t	*sh_fsearch(Shell_t*,const char *,int);
 #endif /* SHOPT_NAMESPACE */
 
+#if SHOPT_MULTIBYTE
+    extern int		sh_isprint(int);
+#else
+#   define sh_isprint(c) isprint(c)
+#endif /* SHOPT_MULTIBYTE */
+
+#define URI_RFC3986_UNRESERVED "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~"
+
 #ifndef ERROR_dictionary
 #   define ERROR_dictionary(s)	(s)
 #endif

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-08-09"
+#define SH_RELEASE	"93u+m 2020-08-10"