Rename fibril_schedule_next_adv() to fibril_switch(). Rename
fibril_schedule_next() to fibril_yield(). Some fibril structures could be
uninitialized, set them to zero in fibril_setup(). For some fibrils, the stack
member can be NULL (e.g. every thread's first/main fibril); don't do free on
these stacks when cleaning up after a dead fibril.