environ(7): Update the description and include some more environment variables

• Add a better introduction to the DESCRIPTION section
• Add a description for MANPATH and POSIXLY_CORRECT
• Asorted improvements for the usage of some macros

PR: 43823
Submitted by: Lyndon Nerenberg <lyndon at orthanc dot ab dot ca>
Reviewed by: 0mp, bcr
Approved by: 0mp, bcr
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25912