Manual pages: Standardize on ENVIRONMENT as section title

Manual pages: Standardize on ENVIRONMENT as section title

There is quite some value (in terms of readability and user
expectations) if consistent names are used for the sections
within manual pages. This patch is one of a series to bring
about this consistency.

But, let's make sure that the section is consistently titled
across pages. Currently we have ENVIRONMENT (many) or ENVIRONMENT
VARIABLES (3).  Let's standardize on ENVIRONMENT (which is also
what is suggested in man-pages(7)).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>