printf %H: fix/reduce encoding into entities (re: 8477d2ce)

The   entity is not valid in XML, only in HTML. Since we must
be compatible with both, it can't be used. Thanks to Andras Farkas
for the bug report.

In addition, the generation of numeric entities for unprintable
characters was only valid while processing UTF-8 text while in a
UTF-8 locale. In all other conditions it produced invalid results.
This is not worth trying to fix.

Discussion:
https://groups.google.com/d/msgid/korn-shell/CAA0nTRta%3DPbOYduyBv%3DXCzumTcUCU8Lki%3DQQf2O8Erk2BFvO1g%40mail.gmail.com

src/cmd/ksh93/bltins/print.c:
- Remove conversion to   entity.
- Remove conversion of non-graph characters to numeric entities.
  Convert only the 5 semantically meaningful characters: < > & " '

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/string.c:
- We don't need sh_isprint() in print.c anymore, so turn it back
  into a static function.

src/cmd/ksh93/tests/builtins.sh:
- Update and trim regression tests.

src/cmd/ksh93/include/defs.h

@@ -446,12 +446,6 @@ 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