Manual pages: Standardize on CONFORMING TO as section title

Manual pages: Standardize on CONFORMING TO 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.

Currently we have STANDARDS (3) or CONFORMING TO (6).
Let's standardize on the latter (which is also what is
suggested in man-pages(7)).

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