Add kqueue support to Janet

Note that this is a work in progress and simply a first attempt at
getting some code into place before being able to test it. This code
follows of sorts both the poll and epoll sections of the codebase hoping
to achieve the exact same.

src/core/state.h

@@ -171,6 +171,11 @@ struct JanetVM {
     int epoll;
     int timerfd;
     int timer_enabled;
+#elif defined(JANET_EV_KQUEUE)
+    JanetHandle selfpipe[2];
+    int kq;
+    int timer;
+    int timer_enabled
 #else
     JanetHandle selfpipe[2];
     struct pollfd *fds;