Add a skeleton framework for capabilities.

Use the first capability to spawn a child process.

zion/include/cap_types.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#define ZC_PROC_SPAWN_CHILD 0x1

zion/include/zcall.h

@@ -12,8 +12,9 @@ uint64_t ZDebug(const char* message);
 
 // TODO: Move structs into an internal header.
 struct ZProcessSpawnReq {
+  uint64_t cap_id;
   uint64_t elf_base;
   uint64_t elf_size;
 };
 
-uint64_t ZProcessSpawn(uint64_t elf_base, uint64_t elf_size);
+uint64_t ZProcessSpawn(uint64_t cap_id, uint64_t elf_base, uint64_t elf_size);

zion/include/zerrors.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#define Z_OK 0x0
+#define ZE_NOT_FOUND 0x1
+#define ZE_INVALID 0x2
+#define ZE_DENIED 0x4