Add libmp.

src/libmp/port/mpmod.c

@@ -0,0 +1,15 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// remainder = b mod m
+//
+// knuth, vol 2, pp 398-400
+
+void
+mpmod(mpint *b, mpint *m, mpint *remainder)
+{
+	mpdiv(b, m, nil, remainder);
+	if(remainder->sign < 0)
+		mpadd(m, remainder, remainder);
+}