_shell_quote: prefer octal.

If we are in a single-byte locale and encounter a non-printable
character we need to escape, previously, if the byte formed a valid
Unicode character, we would print it using \uXXXX notation. For
single-byte characters, use the more compact \XXX octal notation
instead.

Test case:

  LC_ALL=en_US.ISO-8859-1 $SHELL -xc $': \237'

src/mystring.c

@@ -336,8 +336,10 @@ shell_quote(const char *s, int style)
 		} /* } */
 #ifdef WITH_LOCALE
 		if (!iswprint(c) || (c != ' ' && iswblank(c))) {
-			if (c < 128)
+			if (q - p == 1) {
+				c = (unsigned char) *p;
 				goto oct;
+			}
 			fmt = c >= 0x10000 ? "\\U%08x" : "\\u%04x";
 			goto fmt;
 		}