BUGFIX: check parameters for minetest.log lua function

* 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

Fix some crashes and improved behaviour for mod selection gui

Previously, settings where only written when a value has changed, and
removal of a setting value didn't count as a change.

A check for that was there, but was comparing an unsigned variable to
-1, which doesn't work.

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.

by moving return statement out of if-then-else clause...