user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/tests/posix/flockfile.c

@@ -0,0 +1,35 @@
+#include <assert.h>
+#include <time.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <unistd.h>
+
+static void *worker(void *arg) {
+	(void)arg;
+	flockfile(stdout);
+	fputs_unlocked("hello from worker", stdout);
+	funlockfile(stdout);
+	return NULL;
+}
+
+int main() {
+	// Check that recursive locking works.
+	assert(!ftrylockfile(stdout));
+	flockfile(stdout);
+	flockfile(stdout);
+	funlockfile(stdout);
+	funlockfile(stdout);
+	funlockfile(stdout);
+
+	assert(!ftrylockfile(stdout));
+
+	pthread_t thread;
+	int ret = pthread_create(&thread, NULL, &worker, NULL);
+	assert(!ret);
+
+	sleep(1);
+	funlockfile(stdout);
+
+	assert(!pthread_join(thread, NULL));
+	return 0;
+}