[boot] Restructure boot paging and program loading

Restructuring paging into an object that carries its page cache with it
and makes for simpler code. Program loading is also changed to not copy
the pages loaded from the file into new pages - we can impose a new
constraint that anything loaded by boot have a simple, page-aligned
layout so that we can just map the existing pages into the right
addresses. Also included are some linker script changes to help
accommodate this.

src/user/srv.init/init.module

@@ -4,6 +4,7 @@ init = module("srv.init",
     targets = [ "user" ],
     deps = [ "libc", "elf", "bootproto", "zstd" ],
     description = "Init server",
+    ld_script = "init.ld",
     sources = [
         "j6romfs.cpp",
         "loader.cpp",