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/getuid.cpp

@@ -0,0 +1,97 @@
+/*
+ * 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>
+
+namespace Kernel {
+
+uid_t Process::sys$getuid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_uid;
+}
+
+gid_t Process::sys$getgid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_gid;
+}
+
+uid_t Process::sys$geteuid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_euid;
+}
+
+gid_t Process::sys$getegid()
+{
+    REQUIRE_PROMISE(stdio);
+    return m_egid;
+}
+
+int Process::sys$getresuid(uid_t* ruid, uid_t* euid, uid_t* suid)
+{
+    REQUIRE_PROMISE(stdio);
+    if (!validate_write_typed(ruid) || !validate_write_typed(euid) || !validate_write_typed(suid))
+        return -EFAULT;
+    copy_to_user(ruid, &m_uid);
+    copy_to_user(euid, &m_euid);
+    copy_to_user(suid, &m_suid);
+    return 0;
+}
+
+int Process::sys$getresgid(gid_t* rgid, gid_t* egid, gid_t* sgid)
+{
+    REQUIRE_PROMISE(stdio);
+    if (!validate_write_typed(rgid) || !validate_write_typed(egid) || !validate_write_typed(sgid))
+        return -EFAULT;
+    copy_to_user(rgid, &m_gid);
+    copy_to_user(egid, &m_egid);
+    copy_to_user(sgid, &m_sgid);
+    return 0;
+}
+
+int Process::sys$getgroups(ssize_t count, gid_t* user_gids)
+{
+    REQUIRE_PROMISE(stdio);
+    if (count < 0)
+        return -EINVAL;
+    if (!count)
+        return m_extra_gids.size();
+    if (count != (int)m_extra_gids.size())
+        return -EINVAL;
+    if (!validate_write_typed(user_gids, m_extra_gids.size()))
+        return -EFAULT;
+
+    Vector<gid_t> gids;
+    for (auto gid : m_extra_gids)
+        gids.append(gid);
+
+    copy_to_user(user_gids, gids.data(), sizeof(gid_t) * count);
+    return 0;
+}
+
+}