user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,13 @@
+#include <assert.h>
+#include <net/if.h>
+#include <stdio.h>
+
+int main() {
+	char name[IF_NAMESIZE];
+
+	assert(name == if_indextoname(1, name));
+	printf("test: name '%s'\n", name);
+	assert(1 == if_nametoindex(name));
+
+	return 0;
+}