• Sort sample Makefile to comply with the chapter
• Added USE_x examples
• Added option section with examples
• Created new section block, Standard bsd.port.mk Variables

The new section block has always existed, but as a "ghost" section. It is widely
accepted and known, mostly used in the USES section which is quite restrictive.
This added section block will help clear up possible confusion between the USES
section block jumping to the option section and grant the ghost a home.

Reviewed by: mat, matthew (mentor), wblock
Approved by: mat, matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D11523