seperate dirmonitor logic, add build time detection of features (#866)

this also adds libkqueue support

src/api/dirmonitor/kqueue.c

@@ -0,0 +1,53 @@
+#include <sys/event.h>
+#include <sys/stat.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+struct dirmonitor {
+  int fd;
+};
+
+struct dirmonitor* init_dirmonitor_kqueue() {
+  struct dirmonitor* monitor = calloc(sizeof(struct dirmonitor), 1);
+  monitor->fd = kqueue();
+  return monitor;
+}
+
+void deinit_dirmonitor_kqueue(struct dirmonitor* monitor) {
+  close(monitor->fd);
+  free(monitor);
+}
+
+int check_dirmonitor_kqueue(struct dirmonitor* monitor, int (*change_callback)(int, const char*, void*), void* data) {
+  struct kevent event;
+  while (1) {
+    struct timespec tm = {0};
+    int nev = kevent(monitor->fd, NULL, 0, &event, 1, &tm);
+
+    if (nev == -1) {
+      return errno;
+    }
+
+    if (nev <= 0) {
+      return 0;
+    }
+
+    change_callback(event.ident, NULL, data);
+  }
+}
+
+int add_dirmonitor_kqueue(struct dirmonitor* monitor, const char* path) {
+  int fd = open(path, O_RDONLY);
+  struct kevent change;
+
+  EV_SET(&change, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_DELETE | NOTE_EXTEND | NOTE_WRITE | NOTE_ATTRIB | NOTE_LINK | NOTE_RENAME, 0, (void*)path);
+  kevent(monitor->fd, &change, 1, NULL, 0, NULL);
+
+  return fd;
+}
+
+void remove_dirmonitor_kqueue(struct dirmonitor* monitor, int fd) {
+  close(fd);
+}