Fix shellquoting of invalid multibyte char (re: f9d28935, 8c7c60ec)

This commit fixes two bugs in the generation of $'...' shellquoted
strings:
1. A bug introduced in f9d28935. In UTF-8 locales, a byte that is
   invalid in UTF-8, e.g. hex byte 86, would be shellquoted as
   \u[86], which is not the same as the correct quoting, \x86.
2. A bug inherited from 93u+. Single bytes (e.g. hex 11) were
   always quoted as \x11 and not \x[11], even if a subsequent
   character was a hexadecimal digit. However, the parser reads
   past two hexadecimal digits, so we got:
	$ printf '%q\n' $'\x[11]1'
	$'\x111'
	$ printf $'\x111' | od -t x1
	0000000    c4  91
	0000002
   After the bug fix, this works correctly:
	$ printf '%q\n' $'\x[11]1'
	$'\x[11]1'
	$ printf $'\x[11]1' | od -t x1
	0000000    11  31
	0000002

src/cmd/ksh93/sh/string.c: sh_fmtq():
- Make the multibyte code for $'...' more readable, eliminating the
  'isbyte' flag.
- When in a multibyte locale, make sure to shellquote both invalid
  multibyte characters and unprintable ASCII characters as
  hexadecimal bytes (\xNN). This reinstates 93u+ behaviour.
- When quoting bytes, use isxdigit(3) to determine if the next
  character is a hex digit, and if so, protect the quoted byte with
  square brackets.

src/cmd/ksh93/tests/quoting2.sh:
- Move the 'printf %q' shellquoting regression tests here from
  builtins.sh; they test the shellquoting algorithm, not so much
  the printf builtin itself.
- Add regression tests for these bugs.

src/cmd/ksh93/sh/string.c

@@ -40,6 +40,10 @@
 #   define iswprint(c)		(((c)&~0377) || isprint(c))
 #endif
 
+#ifndef isxdigit
+#   define isxdigit(c)		((c)>='0'&&(c)<='9'||(c)>='a'&&(c)<='f'||(c)>='A'&&(c)<='F')
+#endif
+
 
 /*
  *  Table lookup routine
@@ -410,7 +414,6 @@ char	*sh_fmtq(const char *string)
 	}
 	else
 	{
-		int isbyte=0;
 		stakwrite("$'",2);
 		cp = string;
 #if SHOPT_MULTIBYTE
@@ -447,24 +450,29 @@ char	*sh_fmtq(const char *string)
 				break;
 			    default:
 #if SHOPT_MULTIBYTE
-				isbyte = 0;
-				if(c<0)
+				if(mbwide())
 				{
-					c = *((unsigned char *)op);
-					cp = op+1;
-					isbyte = 1;
+					/* We're in a multibyte locale */
+					if(c<0 || c<128 && !isprint(c))
+					{
+						/* Invalid multibyte char, or unprintable ASCII char: quote as hex byte */
+						c = *((unsigned char *)op);
+						cp = op+1;
+						goto quote_one_byte;
+					}
+					if(is_invisible(c))
+					{
+						/* Unicode hex code */
+						sfprintf(staksp,"\\u[%x]",c);
+						continue;
+					}
 				}
-				if(mbwide() && is_invisible(c))
-				{
-					sfprintf(staksp,"\\u[%x]",c);
-					continue;
-				}
-				else if(isbyte)
-#else
+				else
+#endif /* SHOPT_MULTIBYTE */
 				if(!isprint(c))
-#endif
 				{
-					sfprintf(staksp,"\\x%.2x",c);
+				quote_one_byte:
+					sfprintf(staksp, isxdigit(*cp) ? "\\x[%.2x]" : "\\x%.2x", c);
 					continue;
 				}
 				state=0;