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_comutil.c

@@ -70,7 +70,7 @@ zfs_allocatable_devs(nvlist_t *nv)
 boolean_t
 zfs_special_devs(nvlist_t *nv, const char *type)
 {
-	char *bias;
+	const char *bias;
 	uint_t c;
 	nvlist_t **child;
 	uint_t children;
@@ -98,7 +98,7 @@ zpool_get_load_policy(nvlist_t *nvl, zpool_load_policy_t *zlpp)
 {
 	nvlist_t *policy;
 	nvpair_t *elem;
-	char *nm;
+	const char *nm;
 
 	/* Defaults */
 	zlpp->zlp_rewind = ZPOOL_NO_REWIND;