Adds three new linedef types:
Each type adds the tag specified in the tag field to the specified sector's list of tags. It makes overlapping FOF's sooo much nicer to set up.
The attached testing file has two rooms, on of which uses these linedef types and the other which uses normal tags. apply.wad The benefits, I think, speak for themselves.
(In addition, multitagging allocated twice the amount of memory that was necessary, that is, sizeof(list->tags) [a pointer] instead of sizeof(mtag_t) [a 16 bit integer]. I fixed this.)