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/libzpool/util.c

@@ -65,7 +65,7 @@ show_vdev_stats(const char *desc, const char *ctype, nvlist_t *nv, int indent)
 
 	if (desc != NULL) {
 		const char *suffix = "";
-		char *bias = NULL;
+		const char *bias = NULL;
 		char bias_suffix[32];
 
 		(void) nvlist_lookup_uint64(nv, ZPOOL_CONFIG_IS_LOG, &is_log);
@@ -113,12 +113,13 @@ show_vdev_stats(const char *desc, const char *ctype, nvlist_t *nv, int indent)
 
 	for (c = 0; c < children; c++) {
 		nvlist_t *cnv = child[c];
-		char *cname = NULL, *tname;
+		const char *cname = NULL;
+		char *tname;
 		uint64_t np;
 		int len;
 		if (nvlist_lookup_string(cnv, ZPOOL_CONFIG_PATH, &cname) &&
 		    nvlist_lookup_string(cnv, ZPOOL_CONFIG_TYPE, &cname))
-			cname = (char *)"<unknown>";
+			cname = "<unknown>";
 		len = strlen(cname) + 2;
 		tname = umem_zalloc(len, UMEM_NOFAIL);
 		(void) strlcpy(tname, cname, len);
@@ -133,7 +134,7 @@ void
 show_pool_stats(spa_t *spa)
 {
 	nvlist_t *config, *nvroot;
-	char *name;
+	const char *name;
 
 	VERIFY(spa_get_stats(spa_name(spa), &config, NULL, 0) == 0);