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/process.cpp

@@ -0,0 +1,86 @@
+/*
+ * 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 <Kernel/Process.h>
+#include <Kernel/SharedBuffer.h>
+
+namespace Kernel {
+
+pid_t Process::sys$getpid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_pid;
+}
+
+pid_t Process::sys$getppid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_ppid;
+}
+
+int Process::sys$set_process_icon(int icon_id)
+{
+    REQUIRE_PROMISE(shared_buffer);
+    LOCKER(shared_buffers().lock());
+    auto it = shared_buffers().resource().find(icon_id);
+    if (it == shared_buffers().resource().end())
+        return -EINVAL;
+    auto& shared_buffer = *(*it).value;
+    if (!shared_buffer.is_shared_with(m_pid))
+        return -EPERM;
+    m_icon_id = icon_id;
+    return 0;
+}
+
+int Process::sys$get_process_name(char* buffer, int buffer_size)
+{
+    REQUIRE_PROMISE(stdio);
+    if (buffer_size <= 0)
+        return -EINVAL;
+
+    if (!validate_write(buffer, buffer_size))
+        return -EFAULT;
+
+    if (m_name.length() + 1 > (size_t)buffer_size)
+        return -ENAMETOOLONG;
+
+    copy_to_user(buffer, m_name.characters(), m_name.length() + 1);
+    return 0;
+}
+
+int Process::sys$set_process_name(const char* user_name, size_t user_name_length)
+{
+    REQUIRE_PROMISE(proc);
+    if (user_name_length > 256)
+        return -ENAMETOOLONG;
+    auto name = validate_and_copy_string_from_user(user_name, user_name_length);
+    if (name.is_null())
+        return -EFAULT;
+    m_name = move(name);
+    return 0;
+}
+
+}