bugfix(map): Resolve map names that carry either path separator

[bobtista]

• Closes Host's preferred LAN map is not matched when its stored name uses a different separator #3103
• Depends on refactor(lib): Add path separator helpers to PathUtil #3105, whose commit is included here until it merges

MapCache::findMap lowercased the name and then did an exact match against the cache key, so a name
that carried the other path separator never resolved. The map cache keys are built with the platform
separator, while a name can reach the lookup from somewhere that stored the other one, such as a LAN
preference written by a build on a different platform. The lookup silently returns null and the
caller treats the map as absent.

LanGameOptionsMenuInit hit exactly that. It looked the host's preferred map up with a raw lowercased
TheMapCache->find, and on a miss the host slot is never marked as having the map and the map CRC and
size are never set.

Now findMap converts the name's separators to the platform one before matching, so either form
resolves to the same entry. The menu also resolves through findMap and sets the canonical
m_fileName on the game rather than the raw preference string, which keeps one lookup instead of two.

Reproduction:

1. Have a preferred LAN map whose stored name uses / where the map cache key uses \, or the
   reverse. A preference carried over from a build on the other platform does this.
2. Host a LAN game.
3. Observe the host slot is not marked as having the map, and the map CRC and size are not set.

Measured by taking a real cache key, flipping every separator in it, and resolving both forms through
findMap against the same cache:

lookup	before	after
key as stored (...\maps\(3 letter names required)\...)	resolves	resolves
same key with separators flipped (.../maps/(3 letter names required)/...)	does not resolve	resolves

The native form is the control, so a pass is distinguishable from "the cache was empty".

Todo:

• A name carrying the other separator resolves to the same cache entry
• A name carrying the platform separator still resolves, as a control
• Replicate to Generals