utf8: update utf8_is_valid() a bit

Let's avoid a few casts in the function. Also, let's drop the "const"
when returning the string, for similar reasons as strchr() and friends
drop it: so that we don't add a const if the user passes in a non-const
string.

src/basic/utf8.h

@@ -14,7 +14,7 @@
 
 bool unichar_is_valid(char32_t c);
 
-const char *utf8_is_valid(const char *s) _pure_;
+char *utf8_is_valid(const char *s) _pure_;
 char *ascii_is_valid(const char *s) _pure_;
 char *ascii_is_valid_n(const char *str, size_t len);