At present, coroutine stacks must be allocated explicitly by the user,
then initialized with coroutine_stack_init().  This adds a new
coroutine_stack_alloc() function which allocates a stack, making life
easier for users.  coroutine_stack_release() will automatically determine
if the given stack was set up with _init() or alloc() and act
accordingly.

The stacks are allocate with mmap() rather than a plain malloc(), and a
guard page is added, so an overflow of the stack should result in a
relatively debuggable SEGV instead of random data corruption.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>