Fixed map/scenario generation for networked MP campaigns.

The genaration code was inserted before host enters MP Connect mode so that
non-host players would receive an up-to-date level config. This approach
introduces some code duplication, but a better approach would require too much
refactoring and so it doesn't seem suitable for 1.12.

This fixes bug #22307.

N.B. fix follows this suggestion: https://gna.org/bugs/index.php?22307#comment1