Add missing fs.c

src/fs.c

@@ -0,0 +1,21 @@
+#include "fs.h"
+
+struct mg_fd *mg_fs_open(struct mg_fs *fs, const char *path, int flags) {
+  struct mg_fd *fd = (struct mg_fd *) calloc(1, sizeof(*fd));
+  if (fd != NULL) {
+    fd->fd = fs->open(path, flags);
+    fd->fs = fs;
+    if (fd->fd == NULL) {
+      free(fd);
+      fd = NULL;
+    }
+  }
+  return fd;
+}
+
+void mg_fs_close(struct mg_fd *fd) {
+  if (fd != NULL) {
+    fd->fs->close(fd->fd);
+    free(fd);
+  }
+}