bugfix(network): Build map transfer paths with the platform separator

[bobtista]

• Closes Map transfer does not place the received files on non-Windows builds #3102
• Depends on refactor(lib): Add path separator helpers to PathUtil #3105, whose commit is included here until it merges

GetBasePathFromPath and GetFileFromPath search only for '\\', and the six Get*FromMap helpers each rejoin with a literal "\\". Where paths use '/' the base resolves empty and the whole path becomes the file name:

in:      /home/user/GeneralsData/Maps/MyMap/MyMap.map
before:  \/home/user/GeneralsData/Maps/MyMap/MyMap.tga
after:   /home/user/GeneralsData/Maps/MyMap/MyMap.tga

Now the two helpers use getLastPathSeparator, and all six Get*FromMap share one GetFileInMapDirectory that joins with the platform separator.

Todo:

• Transfer a custom map between two clients, both files arrive with matching checksums
• Confirm a Windows build still transfers maps
• Replicate to Generals — N/A, it's in Core