Log Message:
php-fpm: change FPM_USER/FPM_GROUP

pkgsrc changes:
---------------
  * Remove CATEGORIES since it is already included by \ 
${PHPPKGSRCDIR}/Makefile.common.
  * Following a discussion on teck-pkg@ \ 
(https://mail-index.netbsd.org/tech-pkg/2020/04/07/msg022973.html)
    it seems preferable to dissociate the php-fpm user from the one of the Apache web
    server for at least two reasons: (i) www/apache is no longer necessarily the
    standard approach and (ii) running a daemon with a dedicated user seems to \ 
be more
    in line with best practices.