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/zfs/spa_config.c

@@ -245,7 +245,7 @@ spa_write_cachefile(spa_t *target, boolean_t removing, boolean_t postsysevent,
 {
 	spa_config_dirent_t *dp, *tdp;
 	nvlist_t *nvl;
-	char *pool_name;
+	const char *pool_name;
 	boolean_t ccw_failure;
 	int error = 0;
 
@@ -418,7 +418,7 @@ spa_config_generate(spa_t *spa, vdev_t *vd, uint64_t txg, int getstats)
 	unsigned long hostid = 0;
 	boolean_t locked = B_FALSE;
 	uint64_t split_guid;
-	char *pool_name;
+	const char *pool_name;
 
 	if (vd == NULL) {
 		vd = rvd;