Rewrite the QC port of droptofloor()

Rewrite the QC port of droptofloor()

This uses DP_QC_NUDGEOUTOFSOLID to achieve better results (especially
noticeable with sv_legacy_bbox_expand 0) in less CPU time.

Documents the design.
Removes legacy code paths.
Adds warnings for badly placed items.
Implements Quake-compliant behaviour for each BSP format.

See also: https://gitlab.com/xonotic/darkplaces/-/merge_requests/144

Removes hacky workaround for #2774 and thus depends on !1245

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>