Kernel: Move syscall implementations out of Process.cpp

This is something I've been meaning to do for a long time, and here we
finally go. This patch moves all sys$foo functions out of Process.cpp
and into files in Kernel/Syscalls/.

It's not exactly one syscall per file (although it could be, but I got
a bit tired of the repetitive work here..)

This makes hacking on individual syscalls a lot less painful since you
don't have to rebuild nearly as much code every time. I'm also hopeful
that this makes it easier to understand individual syscalls. :^)

Kernel/Syscalls/purge.cpp

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/NonnullRefPtrVector.h>
+#include <Kernel/Process.h>
+#include <Kernel/VM/InodeVMObject.h>
+#include <Kernel/VM/MemoryManager.h>
+#include <Kernel/VM/PurgeableVMObject.h>
+
+namespace Kernel {
+
+int Process::sys$purge(int mode)
+{
+    REQUIRE_NO_PROMISES;
+    if (!is_superuser())
+        return -EPERM;
+    int purged_page_count = 0;
+    if (mode & PURGE_ALL_VOLATILE) {
+        NonnullRefPtrVector<PurgeableVMObject> vmobjects;
+        {
+            InterruptDisabler disabler;
+            MM.for_each_vmobject_of_type<PurgeableVMObject>([&](auto& vmobject) {
+                vmobjects.append(vmobject);
+                return IterationDecision::Continue;
+            });
+        }
+        for (auto& vmobject : vmobjects) {
+            purged_page_count += vmobject.purge();
+        }
+    }
+    if (mode & PURGE_ALL_CLEAN_INODE) {
+        NonnullRefPtrVector<InodeVMObject> vmobjects;
+        {
+            InterruptDisabler disabler;
+            MM.for_each_vmobject_of_type<InodeVMObject>([&](auto& vmobject) {
+                vmobjects.append(vmobject);
+                return IterationDecision::Continue;
+            });
+        }
+        for (auto& vmobject : vmobjects) {
+            purged_page_count += vmobject.release_all_clean_pages();
+        }
+    }
+    return purged_page_count;
+}
+
+}