Fix confused and buggy bridge splashing code

Fix confused and buggy bridge splashing code

A bridge (span or tower) must be splashed when it gets damaged below
SCT_MINEFF.  Likewise when its last supporting sector (bridge head or
tower) gets damaged below SCT_MINEFF, unless EASY_BRIDGES is enabled.
We need to check this whenever a bridge head, span or tower gets
damaged.  This is done in three places, and all of them screw up:

* checksect() ignores damage to bridge heads.  It also leaves writing
  back the sector it checks to the caller, which never happens when
  it's called from sct_postread().

  Note that checksect() drowns all planes on bridges it splashes.
  Functions that need to exempt flying planes from such a fate have to
  splash bridges themselves.

* sect_damage() ignores damage to bridge towers, and damage to bridge
  spans unless EASY_BRIDGES is enabled.  It then runs checksect(),
  which compensates for these omissions, but happily drowns the planes
  sect_damage() attempts to protect.

* eff_bomb() ignores damage to bridge heads.  Collateral damage makes
  sect_damage() run, which compensates for the omission.

This causes the following bugs:

* Efficiency damage going through sect_damage() can drown planes it
  shouldn't.  This affects pinpoint bombing when collateral damage
  splashes a bridge, and strategic bombing.  The drowned planes then
  crash and burn when they attempt to land at their (just splashed)
  base.

* Efficiency damage to bridge heads not going through sect_damage()
  fails to collapse unsupported bridges.  This affects pin-bombing
  efficiency without collateral damage, and ground combat.  Also deity
  commands edit, setsector and add, but that could be regarded as a
  feature.

* If the sector file somehow ends up with an inefficient bridge span,
  it collapses on every read again and again, until it collapses on a
  write.  Related problems exist with other actions of checksect(),
  and they're not addressed here.

* If the sector file somehow ends up with adjacent inefficient bridge
  towers, checksect() on any of them recurses infinitely:

  - checksect() inefficient tower T1
    - knockdown() T1, but don't write that back to the sector file
    - bridgefall() T1; this reads all adjacent sectors, including
      inefficient towert T2
      - checksect() T2
        - knockdown() T2, but don't write that back to the sector file
	- bridgefall() T1; this reads adjacent sectors including T1
	  - checksect() T1
	    ...

This commit creates a new function bridge_damaged() to splash any
bridges that became inefficient or unsupported after damage to a
sector.  To avoid the inifinite recursion, we call it in
sct_prewrite() instead of checksect().

No uses knockdown() outside bridgefall.c remain, so give it internal
linkage.