Kernel: Make purgeable memory a VMObject level concept (again)

This patch changes the semantics of purgeable memory.

- AnonymousVMObject now has a "purgeable" flag. It can only be set when
  constructing the object. (Previously, all anonymous memory was
  effectively purgeable.)

- AnonymousVMObject now has a "volatile" flag. It covers the entire
  range of physical pages. (Previously, we tracked ranges of volatile
  pages, effectively making it a page-level concept.)

- Non-volatile objects maintain a physical page reservation via the
  committed pages mechanism, to ensure full coverage for page faults.

- When an object is made volatile, it relinquishes any unused committed
  pages immediately. If later made non-volatile again, we then attempt
  to make a new committed pages reservation. If this fails, we return
  ENOMEM to userspace.

mmap() now creates purgeable objects if passed the MAP_PURGEABLE option
together with MAP_ANONYMOUS. anon_create() memory is always purgeable.

Kernel/Syscalls/purge.cpp

@@ -19,7 +19,7 @@ KResultOr<FlatPtr> Process::sys$purge(int mode)
     REQUIRE_NO_PROMISES;
     if (!is_superuser())
         return EPERM;
-    int purged_page_count = 0;
+    size_t purged_page_count = 0;
     if (mode & PURGE_ALL_VOLATILE) {
         NonnullRefPtrVector<AnonymousVMObject> vmobjects;
         {