ULTIMA6: Pathfinding fails to detect some doors

[PushmePullyu]

Tested with e28c0d5e37a0d4e83c8ec08e28e12b4ea3f3e725 on Fedora Linux Workstation 40 x86_64

When NPCs do pathfinding as part of their schedule, they fail to open/detect doors if all of the following conditions are true:

1. the door is facing E/W
2. it is closed and unlocked
3. there is an adjacent object S of it

This can be observed when Geoffrey is attempting to path from the castle's dining hall to his bed in the room S of it:
The steel door at 312, 370, 0 will not be detected because there is a chest of drawers at 313, 371, 0 and consequently pathfinding will fail.

To reproduce:

1. Start scummvm with debug logging enabled
2. Load attached enhanced-mode save
3. Press space a few times so Geoffrey will attempt to path to his bed
4. The following message will appear in the log: "actor 7 failed to find a path to 13f,171"

The cause of the problem is an incorrect check in

engines/ultima/nuvie/pathfinder/u6_astar_path.cpp:

sint32 U6AStarPath::step_cost(const MapCoord &c1, const MapCoord &c2)

[PushmePullyu]

PR for this is at ​https://github.com/scummvm/scummvm/pull/6004

[PushmePullyu <127053144+PushmePullyu@…>]

In 09e8ef68:

ULTIMA: NUVIE: Fix pathfinding not detecting some doors

U6 NPCs following their schedule can open doors in the way.
However, E/W facing doors would not be detected if an object was
directly S of them, causing pathing to fail in some cases.

Fix and simplify door detection.

Closes #15308