nvpair: Constify string functions

After addressing coverity complaints involving `nvpair_name()`, the
compiler started complaining about dropping const. This lead to a rabbit
hole where not only `nvpair_name()` needed to be constified, but also
`nvpair_value_string()`, `fnvpair_value_string()` and a few other static
functions, plus variable pointers throughout the code. The result became
a fairly big change, so it has been split out into its own patch.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14612

module/zcommon/zfs_deleg.c

@@ -102,9 +102,9 @@ zfs_deleg_canonicalize_perm(const char *perm)
 }
 
 static int
-zfs_validate_who(char *who)
+zfs_validate_who(const char *who)
 {
-	char *p;
+	const char *p;
 
 	if (who[2] != ZFS_DELEG_FIELD_SEP_CHR)
 		return (-1);