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/zcp_get.c

@@ -391,7 +391,7 @@ get_special_prop(lua_State *state, dsl_dataset_t *ds, const char *dsname,
 		dsl_dataset_crypt_stats(ds, nvl);
 		if (nvlist_lookup_nvlist(nvl, zfs_prop_to_name(zfs_prop),
 		    &propval) == 0) {
-			char *source;
+			const char *source;
 
 			(void) nvlist_lookup_uint64(propval, ZPROP_VALUE,
 			    &numval);