Zpool upgrade returns error messages to stdout instead of stderr

Zpool upgrade returns error messages to stdout instead of stderr:

1. In function:
```C
static int
upgrade_version(zpool_handle_t *zhp, uint64_t version)
```

Following code currently returns error message to stdout:
```C
	if (unsupp_fs) {
		(void) printf(gettext("Upgrade not performed due to %d "
		    "unsupported filesystems (max v%d).\n"),
		    unsupp_fs, (int) ZPL_VERSION);
		return (1);
	}
```
Also in function:
```C
static int
upgrade_one(zpool_handle_t *zhp, void *data)
```

Following code currently returns error message to stdout:
```C
 	if (strcmp("log", zpool_get_name(zhp)) == 0) {
		(void) printf(gettext("'log' is now a reserved word\n"
		    "Pool 'log' must be renamed using export and import"
		    " to upgrade.\n"));
		return (1);
	}
```

We can take a simple test (note: log is my pool name):

1. Redirect stdout to /dev/null
```shell
zpool upgrade log 1> /dev/null
```

2. Redirect stderr to /dev/null
```shell
zpool upgrade log 2> /dev/null
This system supports ZFS pool feature flags.

'log' is now a reserved word
Pool 'log' must be renamed using export and import to upgrade.
```