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

this also adds libkqueue support

src/api/dirmonitor/inotify.c

@@ -0,0 +1,58 @@
+#include <sys/inotify.h>
+#include <limits.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <fcntl.h>
+
+struct dirmonitor {
+  int fd;
+};
+
+struct dirmonitor* init_dirmonitor_inotify() {
+  struct dirmonitor* monitor = calloc(sizeof(struct dirmonitor), 1);
+
+  monitor->fd = inotify_init();
+  fcntl(monitor->fd, F_SETFL, O_NONBLOCK);
+
+
+  return monitor;
+}
+
+void deinit_dirmonitor_inotify(struct dirmonitor* monitor) {
+  close(monitor->fd);
+  free(monitor);
+}
+
+int check_dirmonitor_inotify(struct dirmonitor* monitor, int (*change_callback)(int, const char*, void*), void* data) {
+  char buf[PATH_MAX + sizeof(struct inotify_event)];
+  ssize_t offset = 0;
+
+  while (1) {
+    ssize_t len = read(monitor->fd, &buf[offset], sizeof(buf) - offset);
+
+    if (len == -1 && errno != EAGAIN) {
+      return errno;
+    }
+
+    if (len <= 0) {
+      return 0;
+    }
+
+    while (len > sizeof(struct inotify_event) && len >= ((struct inotify_event*)buf)->len + sizeof(struct inotify_event)) {
+      change_callback(((const struct inotify_event *)buf)->wd, NULL, data);
+      len -= sizeof(struct inotify_event) + ((struct inotify_event*)buf)->len;
+      memmove(buf, &buf[sizeof(struct inotify_event) + ((struct inotify_event*)buf)->len], len);
+      offset = len;
+    }
+  }
+}
+
+int add_dirmonitor_inotify(struct dirmonitor* monitor, const char* path) {
+  return inotify_add_watch(monitor->fd, path, IN_CREATE | IN_DELETE | IN_MOVED_FROM | IN_MOVED_TO);
+}
+
+void remove_dirmonitor_inotify(struct dirmonitor* monitor, int fd) {
+  inotify_rm_watch(monitor->fd, fd);
+}