print command needs better handling for string output

[sdimitro]

Currently:

sdb> spa test | vdev | filter obj.vdev_log_mg > 0 | member vdev_path | print -Rc
0xffff9078c070b8a0 = "/dev/xvdb1"

We don't want to have the address printed for the char * - it should be:

sdb> spa test | vdev | filter obj.vdev_log_mg > 0 | member vdev_path | print -Rc
"/dev/xvdb1"

And maybe it would be a good idea to provide an option to unquote strings and characters:

sdb> spa test | vdev | filter obj.vdev_log_mg > 0 | member vdev_path | print -Rc <+ new argument>
/dev/xvdb1

[sdimitro]

cc @jwk404