kernel: separate headers into kernel and user headers

all kernel headers have been moved from include/mango to include/kernel
and include definitions that are only relevant to kernel-space.

any definitions that are relevant to both kernel- and user-space
(i.e. type definitions, syscall IDs) have been moved to
include/mango within libmango.

CMakeLists.txt

@@ -40,6 +40,7 @@ add_executable(${kernel_exe_name}
 target_include_directories(${kernel_exe_name} PRIVATE
 	include
 	libc/include
+	libmango/include
 	arch/${kernel_arch}/include)
 target_compile_options(${kernel_exe_name} PRIVATE
 	-nostdlib -ffreestanding