ensure calling order on loaded modules

- IModule changed to contain module destructor
- Instance methods on modules are now guaranteed to be called in loading
  order
documented building and installation
better formatting of instructions
add warning to xtracks help about cbmdos

- xtracks -X option must be called AFTER loading cbmdos, document this
link-time optimizations for GCC release builds
complete 'install' target, better configurability
prepare possibility to ask back reserved blocks
block method to get module reserving this block
bugfix: pointer initialization was missing
IBlockAllocator and diskfile writing changes

- make use of possibility to ask modules for reserved blocks
cbmdos: allow to use more directory blocks

- new parameter for how many directory blocks to reserve initially
- allow allocating more blocks as needed
cbmdos: implement giving reserved blocks back
cbmdos: fix interleave in _nextDirBlock()

- force interleave when track changed
pushed versions for 0.4b
Merge branch 'master' into release