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/readlink.cpp

@@ -0,0 +1,68 @@
+/*
+ * 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/StringView.h>
+#include <Kernel/FileSystem/FileDescription.h>
+#include <Kernel/FileSystem/VirtualFileSystem.h>
+#include <Kernel/Process.h>
+
+namespace Kernel {
+
+int Process::sys$readlink(const Syscall::SC_readlink_params* user_params)
+{
+    REQUIRE_PROMISE(rpath);
+
+    Syscall::SC_readlink_params params;
+    if (!validate_read_and_copy_typed(&params, user_params))
+        return -EFAULT;
+
+    if (!validate(params.buffer))
+        return -EFAULT;
+
+    auto path = get_syscall_path_argument(params.path);
+    if (path.is_error())
+        return path.error();
+
+    auto result = VFS::the().open(path.value(), O_RDONLY | O_NOFOLLOW_NOERROR, 0, current_directory());
+    if (result.is_error())
+        return result.error();
+    auto description = result.value();
+
+    if (!description->metadata().is_symlink())
+        return -EINVAL;
+
+    auto contents = description->read_entire_file();
+    if (contents.is_error())
+        return contents.error();
+
+    auto& link_target = contents.value();
+    auto size_to_copy = min(link_target.size(), params.buffer.size);
+    copy_to_user(params.buffer.data, link_target.data(), size_to_copy);
+    // Note: we return the whole size here, not the copied size.
+    return link_target.size();
+}
+
+}