sysutils/zfsnap2: must install periodic script in relevant directory.

Previously they were installed to %%EXAMPLESDIR%%. This commit also
combines the create and delete scripts into one, and fixes a bug in the
delete script.

Upstream PR: https://github.com/zfsnap/zfsnap/pull/112

PR: 221689
Reported by: Anton Saietskii <vsasjason@gmail.com>
Approved by: dmgk (ports) mm (maintainer timeout)
Sponsored by: Axcient