user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,20 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+
+int main() {
+	int res;
+
+	res = system("true");
+	assert(WIFEXITED(res));
+	assert(WEXITSTATUS(res) == 0);
+
+	res = system("false");
+	assert(WIFEXITED(res));
+	assert(WEXITSTATUS(res) == 1);
+
+	res = system(NULL);
+	assert(res == 1);
+
+	return 0;
+}