Fix crash when no world is selected and configure button is pressed.

by moving return statement out of if-then-else clause...
Improve behaviour for empty modpacks and when no mods at all are installed:

Only show enable all / disable all buttons for all add-ons when at
least one add-on is installed. When no add-on ist installed, don't
show any buttons or checkboxes.

Added is_modpack flag to ModSpec to distinguish empty modpacks from
normal mods and check this flag in mod selection gui so that empty
modpacks are not treated like mods that can be enabled or disabled.
Fix crash when pressing delete button in server browser and no server is selected.

A check for that was there, but was comparing an unsigned variable to
-1, which doesn't work.
Make sure that settings are written to config file when settings are removed.

Previously, settings where only written when a value has changed, and
removal of a setting value didn't count as a change.
Treegen update. Some new symbols. Speed up code a bit.
Tweak buildbot

* CURL support
* only download packages if they don't exist
* fixed download links (libvorbis and libogg)
* manual downloading if automatic downloading fails (only minetest and minetest_game)
* mingwm10.dll and openal_stripped.zip are downloaded automatically
Added "force_load" field for entities, which makes them load the world around them like a player.