Enhance the multitagging functionality added by !1485

I suggested some enhancements in a comment on !1485 (merged), and decided to code it myself. Here's what I added:

• New linedef special 96: "Apply Tag to Tagged Sectors"

  • Applies the tag of the linedef's front sector to all tagged sectors.
  • Target tags are given by the linedef's tag, and optionally its texture offsets, for a maximum of five tags per linedef.
  • Effect 6 = Use Front Side Offsets (if nonzero)
  • Transfer Line = Use Back Side Offsets (if nonzero)

• New flags for linedef specials 97-99

  • Like with 96, the "Effect 6" and "Transfer Line" flags enable the front and back texture offsets respectively, though in this case they are used as additional tags to apply rather than target tags.

I've attached a WAD for testing the new functionality, and as far as I can tell, it works as intended: apply2.wad