devel/py-libzfs: Update GH_ACCOUNT, fix build on legacy ZFS

Make lzc_wait conditional on support in ZFS, since it is a relatively
recent addition.

While here, update GH_ACCOUNT to truenas as suggested by diizzy.

Reviewed by: kbowling
Approved by: kbowling (ports)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D31394