additional way of clipping areas: tagclips

In addition to the existing clip- and plclip-entities, this patch provides
a new approach to provide the same: clip areas by tagging cubes.
Each cube of a map has a parameter "tag". All tags in all existing ac maps
are currently zero. It is a leftover from cube 1. This patch uses the tag value
to determine, if a cube (from floor to ceiling) should be forbidden for
players (tag 0x80) or every moving object (tag 0x40). Tagclips have several
advantages over classic clip entities: they are fast to establish (mark
area, then /edittag 0x80) and are "cheap" for the engine. Tagclips do not
increase the number of entities. As tagclips are always floor to ceiling,
they can't be used to build stairs or platforms. Tagclips should be
preferred over clip entities, but can coexist without problems.

Technically, setting the tag value to 0x40 or 0x80 causes the physics
engine to handle the cube like a "solid".