rsolve: use limit() to substitute initial values of n in the generic solution of the recurrence equation

Using just subs() rsolve sometimes is not able to find solution with specified initial conditions.

An example in master:
```
In [1]: f = (-16*n**2 + 32*n - 12)*y(n - 1) + (4*n**2 - 12*n + 9)*y(n)

In [2]: rsolve(f, y(n), {y(1): binomial(2*n + 1, 3)})
Out[2]: 0

In [3]: g=rsolve(f, y(n))

In [4]: g.subs(n,1)
Out[4]: ∞⋅C₀
In [5]: g
Out[5]: 
 -n  n - 1              
2  ⋅8     ⋅C₀⋅(n - 1/2)!
────────────────────────
       (n - 3/2)!       
In [6]: g.limit(n,1)
Out[6]: 
C₀
──
4 
```