[kernel] Expose a sysconf page to userspace

A structure, system_config, which is dynamically defined by the
definitions/sysconf.yaml config, is now mapped into every user address
space. The kernel fills this with information about itself and the
running machine.

User programs access this through the new j6_sysconf fake syscall in
libj6.

See: Github bug #242
See: [frobozz blog post](https://jsix.dev/posts/frobozz/)

Tags:

src/kernel/vm_space.cpp

@@ -9,6 +9,7 @@
 #include "objects/process.h"
 #include "objects/thread.h"
 #include "objects/vm_area.h"
+#include "sysconf.h"
 #include "vm_space.h"
 
 // The initial memory for the array of areas for the kernel space
@@ -46,6 +47,14 @@ vm_space::vm_space() :
     memset(m_pml4, 0, mem::frame_size/2);
     for (unsigned i = arch::kernel_root_index; i < arch::table_entries; ++i)
         m_pml4->entries[i] = kpml4->entries[i];
+
+    // Every vm space has sysconf in it
+    vm_area *sysc = new vm_area_fixed(
+            g_sysconf_phys,
+            sizeof(system_config),
+            vm_flags::none);
+
+    add(sysconf_user_address, sysc);
 }
 
 vm_space::~vm_space()