Set up multiplayer games started from the commandline equivalently to lobby games.
This means that parameters now use the correct default values, scenarios using
random maps work and side leaders always have unique IDs.
Scenario and side parameters are defined as follows, in order of increasing priority:
  Wesnoth defaults --> map settings --> commandline-specified values
All modifications are in a code path separate from lobby games (as it was before),
so lobby games are not affected.
This fixes bugs #19853, #19883, #19895 and #20009.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56070 75b3775d-3801-0410-9e12-e2e43121521d