ctfmerge.1: Import ENVIRONMENT from NetBSD

Import the ENVIRONMENT section from NetBSD, minus the variable that our
ctfmerge does not have. Alphabetize them, polish grammar and alignment,
and add the variables to the man database. While here, remove whitespace
from the end of some lines to quiet linter.

MFC after: 3 days
PR: 291186
Co-authored-by: Alexander Ziaee <ziaee@FreeBSD.org>
Obtained from: NetBSD (christos <christos@NetBSD.org>, 8a0c0d8)
Differential Revision: https://reviews.freebsd.org/D54054