Rename util/list.h to system/list.h

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Rename __libsystem_* attribute definitions to __sys_*

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Remove unused thread member from user_timer_t

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Rename TIMER_EVENT_FIRED to just TIMER_EVENT

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Add kernel semaphore API

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>