bsd.incs.mk: Add support to override tags for a specific include group

With this patch if a Makefile is using the INCSGROUPS mechanisms it can
override the default package for specific includes files using
GROUPPACKAGE= mynewpackage

While here add a few comments after endif/endfor so it's easier to read.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D27611