New lua metatable "wesnoth.mp_settings".
Follows pattern of previous lua hooks in src/scripting/lua.cpp.

I would like to be able to access all the fields that occur in
"multiplayer" tag in save game files.

Referring to gamestatus.cpp line 1208, which is what writes
the game state to a config for output, I check if

(game_state.classification().campaign_type=="multiplayer" ),

and if so I ask the mp_settings object for fields.

Referring to line 195 of mp_game_settings.cpp, I use the same names
as appear in save files for the lua hooks.

I don't currently handle vector objects like "active_mods".
enable wesnoth.mp_settings.active_mods and side_users
(using string join functions as to_config() does)
make campaign_type visible from wesnoth.game_config
fix int mp_countdown -> bool, oops
delete explicit attribute references, write a cfg instead
remove now unnecesary include
moved mp_settings to be a (conditional) child of game_config,
using return_cfgref_attrib fcn.
works fine in testing, using path wesnoth.game_config.mp_settings
Fix a goto jumping past va_end

Found by coverity
Slightly tweak multiplayer lobby icon spacings for advertized games.
Move multiplayer lobby map size info to bottom icon area.
Make multiplayer lobby advertized game names and vacant slots bold.
MP lobby advertized game layout changes.
MP lobby: hide map icon in the event the map is not loadable.
SotBE S1: add bowmen to enemy AI recruit list
SotBE S3: clarify objectives

Indicate that it is good enough to accomplish either one of the
objectives.
SotBE: add side parameter to orcish shaman macros

In S4 they now start on Side 3, not Side 1.
SotBE S4: change the map to make scenario more interesting

Kapou'e now has to go through the Side 5 elves to get to the center
keep with the shamans.
SotBE S4: remove two pieces of dialog

They are unnecessary or don't apply to the new map any more.
SotBE S4: move shamans to Side 1 at end of scenario
SotBE S4: give the player control of side 3 orcs
SotBE S4: balancing adjustments to gold, turns, ...

... income, recruit lists, AI parameters
Added campaign difficulty info for game item in MP Lobby.
Move format_time_summary to its own file to fix linking issues