Fix and improve get_pak.sh

[ceeac]

This patch fixes and improves get_pak.sh. Most notable changes:

• Fixed failure to install zipped paksets which only contain a simutrans/ folder at the root (e.g. pak64)
• Added support for installing paksets from .cab and .tar.gz files (pak128.japan and pak64.h0scale)
• Updated link to newest version of pak64.german (old link was dead)
• Added progress bar for curl/wget downloads
• Added check for correct installation directory (must end with /simutrans)

I have only tested this on Linux, I do not know if it works on mingw/cygwin.

[prissi]

There was a return in line 214 which should have been an exit. Otherwise it seems to work. In it r8933