Throw const on some strings

In C, const indicates to the reader that mutation will not occur.
It can also serve as a hint about ownership.

Add const in a few places where it makes sense.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10997

module/unicode/u8_textprep.c

@@ -330,7 +330,7 @@ const uint8_t u8_valid_max_2nd_byte[0x100] = {
  * specific to UTF-8 and Unicode.
  */
 int
-u8_validate(char *u8str, size_t n, char **list, int flag, int *errnum)
+u8_validate(const char *u8str, size_t n, char **list, int flag, int *errnum)
 {
 	uchar_t *ib;
 	uchar_t *ibtail;