stand: Change zfs_parsedev() API

Change the first argument to zfs_parsedev() to be a pointer to a struct
devdesc *. This now gets filled in with a malloc'd structure that's
returned to the caller that the caller is repsonsible for freeing. Most
nplaces in the tree passed in a malloc'd pointer anyway, and this moves
knowledge of zfs_devdesc more firmly into the zfs.c code.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37336