Change functions which return literals to return const char*

get_format_prompt_string() and zpool_state_to_name() return
a string literal which is read-only, thus they should return
`const char*`.

zpool_get_prop_string() returns a non-const string after
successful nv-lookup, and returns a string literal otherwise.
Since this function is designed to be used for read-only purpose,
the return type should also be `const char*`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7285

lib/libzfs/libzfs_crypto.c

@@ -130,7 +130,7 @@ catch_signal(int sig)
 	caught_interrupt = sig;
 }
 
-static char *
+static const char *
 get_format_prompt_string(zfs_keyformat_t format)
 {
 	switch (format) {

lib/libzfs/libzfs_pool.c

@@ -113,7 +113,7 @@ zpool_props_refresh(zpool_handle_t *zhp)
 	return (0);
 }
 
-static char *
+static const char *
 zpool_get_prop_string(zpool_handle_t *zhp, zpool_prop_t prop,
     zprop_source_t *src)
 {
@@ -181,7 +181,7 @@ zpool_get_prop_int(zpool_handle_t *zhp, zpool_prop_t prop, zprop_source_t *src)
 /*
  * Map VDEV STATE to printed strings.
  */
-char *
+const char *
 zpool_state_to_name(vdev_state_t state, vdev_aux_t aux)
 {
 	switch (state) {