Added specific interrupt handler for INT_SYSCALL

System calls are done via interruptions; this code enables them
by creating a interrupt handler that will end the call.
Switch address spaces when scheduling a thread of another task

Class method Task::activate() loads in CR3 (or PDBR) its address
space. We do this only when the tasks are different, since a
write to CR3 causes flushing of some paging-related data structures.

Notice that we moved the method Thread::dispatch to the thread.cc
to avoid circular dependencies between Thread and Task.
Added a constant 'identified' in all traits headers that lacked one.
Add TypeSum metaprogram.
Replace all the init_stack overloads by a single variadic template.
Replace all constructors of Adapter with a single variadic constructor.
Replace LIST metaprogram with a truly variadic one.
Replaced the constructors in Handle with a single variadic constructor.
Replaced Message::in and Message::out overlods with variadic functions.
Replaced the constructors of Stub with variadic ones.