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

lib/libzutil/os/linux/zutil_import_os.c

@@ -168,9 +168,9 @@ zpool_open_func(void *arg)
 	 * Add additional entries for paths described by this label.
 	 */
 	if (rn->rn_labelpaths) {
-		char *path = NULL;
-		char *devid = NULL;
-		char *env = NULL;
+		const char *path = NULL;
+		const char *devid = NULL;
+		const char *env = NULL;
 		rdsk_node_t *slice;
 		avl_index_t where;
 		int timeout;
@@ -769,7 +769,7 @@ no_dev:
  *    vdev_enc_sysfs_path: '/sys/class/enclosure/11:0:1:0/SLOT 4'
  */
 static void
-update_vdev_config_dev_sysfs_path(nvlist_t *nv, char *path)
+update_vdev_config_dev_sysfs_path(nvlist_t *nv, const char *path)
 {
 	char *upath, *spath;
 
@@ -795,7 +795,7 @@ sysfs_path_pool_vdev_iter_f(void *hdl_data, nvlist_t *nv, void *data)
 {
 	(void) hdl_data, (void) data;
 
-	char *path = NULL;
+	const char *path = NULL;
 	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_PATH, &path) != 0)
 		return (1);
 
@@ -841,7 +841,7 @@ void
 update_vdev_config_dev_strs(nvlist_t *nv)
 {
 	vdev_dev_strs_t vds;
-	char *env, *type, *path;
+	const char *env, *type, *path;
 	uint64_t wholedisk = 0;
 
 	/*