stand: Introduce devparse to parse device / path strings

devparse is now the preferred interface to use to parse device
strings or device:/path strings. It parses the passed in string,
mallocs the device's particular devdesc string and returns the
'remainder' of the device:/path for further processing.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37338

(cherry picked from commit 781ca0afcd6c64ac3fbaf2a19b1880734cdfe947)