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