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/include/janet.h

@@ -198,6 +198,11 @@ extern "C" {
 #define JANET_EV_EPOLL
 #endif
 
+/* TODO: Probably breaks NetBSD, might need help here. */
+#if defined(JANET_BSD) && !defined(JANET_EV_NO_KQUEUE)
+#define JANET_EV_KQUEUE
+#endif
+
 /* How to export symbols */
 #ifndef JANET_API
 #ifdef JANET_WINDOWS