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

cmd/zdb/zdb.c

@@ -4209,7 +4209,7 @@ collect_nvlist_stats(nvlist_t *nvl, zdb_nvl_stats_t *stats)
 {
 	nvlist_t *list, **array;
 	nvpair_t *nvp = NULL;
-	char *name;
+	const char *name;
 	uint_t i, items;
 
 	stats->zns_list_count++;