Replace COSMO define with _COSMO_SOURCE

This change might cause ABI breakages for /opt/cosmos. It's needed to
help us better conform to header declaration practices.

libc/calls/syncfs.c

@@ -0,0 +1,37 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/calls/syscall-sysv.internal.h"
+
+/**
+ * Syncs filesystem associated with file descriptor.
+ *
+ * Since Linux 5.8, syncfs() will report an error if an inode failed to
+ * be written during the time since syncfs() was last called.
+ *
+ * @return 0 on success, or -1 w/ errno
+ * @raise EIO if some kind of data/metadata write error happened
+ * @raise ENOSPC if disk space was exhausted during sync
+ * @raise EDQUOT (or ENOSPC) on some kinds of NFS errors
+ * @raise EBADF if `fd` isn't a valid file descriptor
+ * @raise ENOSYS on non-Linux
+ */
+int syncfs(int fd) {
+  return sys_syncfs(fd);
+}