bsdinstall(8): Document ZFS installation scripting.

bsdinstall(8): Document ZFS installation scripting.

- Extend the bsdinstall(8) man page with ZFS installation scripting
  details. [1]
- Extend the bsdinstall(8) man page with the description of all the ZFS
  variables involved in a scripted installation of ZFS-based systems. [1]
- Extend the SCRIPTING section with an example for a ZFS-based scripted
  installation. [1]
- Create a new section explaining how ZFS datasets must be written into
  a variable to get them set on the final system. [1]

While here:
- Add Roberto to the copyrights for recognition as changes to the manual
  page are huge.
- Use "Dq" for default values.
- Use sysrc(8) instead of echo in examples.

Submitted by:	Roberto Fernandez Cueto <roberfern@gmail.com> [1]
Reviewed by:	dteske
Approved by:	re (gjb), krion (mentor, implicit), mat (mentor, implicit)
Differential Revision:	https://reviews.freebsd.org/D14169