sysutils/zfsnap: fix -s/-S mode.

Move creation of files from "do-patch" to "post-extract" target,
so that patches from files/ directory get applied.

While here, add LICENSE.

PR: 223951
Submitted by: Anton Sayetsky <vsasjason@gmail.com> (based on)
Approved by: yamagi@yamagi.org (maintainer timeout, 7 weeks)