Cut currdev dev and path fields for ZFS on the last colon.

Previously the code cut those fields on second colon, that prevented
boot from boot environments with colon in their names. This change
moves the limitation from dev field to path, which is empty by default.

Reviewed by: allanjude, tsoome
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D16068