Implement memory area handling, for mmap-like memory management

In the current kernel design, memory management is handle by a very minimalistic abstraction, without any support for a 'memory area' representation. This design was excellent in the early stage of the kernel development, but make mmap(), shared memory and auto-growing stack difficult to implement on top of it.

A new approach is needed, with an implementation of memory segment, more or less close to mmap needs. It will allow to have a growing stack and should simplify process creation, ELF loader, and heap management!