[kernel] Improve process init

Move process init from each process needing a main.s with _start to
crt0.s in libc. Also change to a sysv-like initial stack with a
j6-specific array of initialization values after the program arguments.

src/include/j6/init.h

@@ -0,0 +1,30 @@
+#pragma once
+/// \file init.h
+/// Types used in process and thread initialization
+
+#include <stdint.h>
+
+enum j6_init_type {					// `value` is a:
+	j6_init_handle_system,			// Handle to the system
+	j6_init_handle_process,			// Handle to this process
+	j6_init_handle_thread,			// Handle to this thread
+	j6_init_handle_space,			// Handle to this process' address space
+	j6_init_desc_framebuffer		// Pointer to a j6_init_framebuffer descriptor
+};
+
+struct j6_init_value {
+	enum j6_init_type type;
+	uint64_t value;
+};
+
+/// Structure defining a framebuffer.
+/// `flags` has the following bits:
+///   0-3:   Pixel layout. 0000: bgr8, 0001: rgb8
+struct j6_init_framebuffer {
+	void* addr;
+	size_t size;
+	uint32_t vertical;
+	uint32_t horizontal;
+	uint32_t scanline;
+	uint32_t flags;
+};