[kernel] First steps at removing channel objects

This commit does a number of things to start the transition of channels
from kernel to user space:

- Remove channel objects / syscalls from the kernel
- Add mutex type in libj6
- Add condition type in libj6
- Add a `ring` type flag for VMA syscalls to create ring buffers
- Implement a rudimentary shared memory channel using all of the above

src/libraries/j6/condition.cpp

@@ -0,0 +1,37 @@
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <j6/condition.hh>
+#include <j6/errors.h>
+#include <j6/syscalls.h>
+#include <j6/syslog.hh>
+
+namespace j6 {
+
+void
+condition::wait()
+{
+    j6::syslog("Waiting on condition %lx", this);
+    uint32_t v = __atomic_add_fetch(&m_state, 1, __ATOMIC_ACQ_REL);
+    j6_status_t s = j6_futex_wait(&m_state, v, 0);
+    while (s == j6_status_futex_changed) {
+        v = m_state;
+        if (v == 0) break;
+        s = j6_futex_wait(&m_state, v, 0);
+    }
+    j6::syslog("Woke on condition %lx", this);
+}
+
+void
+condition::wake()
+{
+    uint32_t v = __atomic_exchange_n(&m_state, 0, __ATOMIC_ACQ_REL);
+    if (v)
+        j6_futex_wake(&m_state, 0);
+}
+
+
+} // namespace j6
+
+#endif // __j6kernel