Auto travel is unreliable in forests while riding a horse

# Describe the bug
Auto travel tries to go through things that would be passable on foot but not mounted regardless of whether the player is mounted.

# Steps To Reproduce
Steps to reproduce the behavior:
1. Use auto travel through a stretch of forest while riding a horse

# Expected behavior
Auto travel is aware of what is passable in the player's current mode of travel.

# Versions and configuration
- OS: Linux
- OS Version: Kernel 5.4.5-arch1-1
- Game Version: 0.D-10565-g1e8f27e [64-bit]
- Graphics Version: Tiles
- Mods loaded: [
Dark Days Ahead [dda],
Disable NPC Needs [no_npc_food],
No Fictional Guns [no_rivtech_guns],
No Flaming Weapons [no_flaming_weapons]
]

No source file, test, or entry point is named in the issue. First reproduce auto travel through forest while riding a horse, then trace the auto-travel pathfinding logic and its passability checks; done means mounted travel avoids terrain that is only passable on foot, while unmounted travel remains unaffected.

Written by the indexing model from the issue text.