Kernel: Add a systrace() syscall and implement /bin/strace using it.

Calling systrace(pid) gives you a file descriptor with a stream of the
syscalls made by a peer process. The process must be owned by the same
UID who calls systrace(). :^)

Kernel/ProcessTracer.cpp

@@ -0,0 +1,31 @@
+#include <Kernel/ProcessTracer.h>
+#include <AK/kstdio.h>
+
+ProcessTracer::ProcessTracer(pid_t pid)
+    : m_pid(pid)
+{
+
+}
+
+ProcessTracer::~ProcessTracer()
+{
+
+}
+
+void ProcessTracer::did_syscall(dword function, dword arg1, dword arg2, dword arg3, dword result)
+{
+    CallData data = { function, arg1, arg2, arg3, result };
+    m_calls.enqueue(data);
+}
+
+int ProcessTracer::read(byte* buffer, int buffer_size)
+{
+    if (!m_calls.is_empty()) {
+        auto data = m_calls.dequeue();
+        // FIXME: This should not be an assertion.
+        ASSERT(buffer_size == sizeof(data));
+        memcpy(buffer, &data, sizeof(data));
+        return sizeof(data);
+    }
+    return 0;
+}