Make kern_object_type() behave like all other syscalls (return status_t)

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Hide private kernel/libkernel definitions unless __KERNEL_PRIVATE is defined

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Nuke POSIX signal implementation from kernel

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