* 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

Wireshark dissector: Add TOSERVER_RESPAWN and TOCLIENT_DEATHSCREEN, plus some failed assertion fix (well, hack).

minetestmapper: Fix the way ylist is computed, use cStringIO instead of rolling own Bytestream, remove some artifacts when drawing underground blocks, introduce a --drawunderground switch (defaults to of), 'blocknum' renamed to 'content'.

If it doesn't find colors.txt locally, it looks for the one in the
directory of the script itself.

This solves two issues at once:
* CMake would delete po files during ‘make clean’ because it thought
  they were autogenerated and not just managed
* the only gettext tools readily available in Windows are so old they
  don't support options like --package-name

The change also moves minetest.pot down one level, so we don't need to
special case ‘en’ anymore.

The downside is of course that you need some sane POSIX shell to update
the po files.