initial commit of prbl 1061

Created Makefile and date.c.
finish gcd
finish bezout (Euclidean algorithm)
finish modulus inverse
finish flinear congruence equation solver
finish whole program
fix negative solution bug

Previously, in the function a0 x = b0 (mod n), a0 is guaranteed to be
positive, but b0 was neglected. b0 also needs to be positive (actually
better in the range [1, n-1]) to assure that (x0 * b0) % n is positive.