user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,30 @@
+#include <assert.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+void prevent_atforks(void){
+	exit(1);
+}
+
+int main() {
+	pid_t pid;
+	int status;
+	assert(!pthread_atfork(prevent_atforks, NULL, NULL));
+
+	switch (pid = vfork()) {
+	case -1:
+		perror("vfork");
+		abort();
+	case 0:
+		_exit(12);
+	default: break;
+	}
+
+	assert(wait(&status) == pid);
+	assert(WIFEXITED(status) && WEXITSTATUS(status) == 12);
+
+	return 0;
+}