7803 want devid_str_from_path(3devid)

illumos/illumos-gate@46d46cd4fa12218e54fa3d73a9be825ed504cabb
https://github.com/illumos/illumos-gate/commit/46d46cd4fa12218e54fa3d73a9be825ed504cabb

https://www.illumos.org/issues/7803

Make get_devid() from libzfs a public function in libdevid, as its pretty
usable in other places and duplicating all the logic required to get string
encoded devid from path seems counter-productive.

Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Jason King <jason.brian.king@gmail.com>
Reviewed by: Marcel Telka <marcel@telka.sk>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>