pbinom accuracy (PR#6757)

Full_Name: Morten Welinder
Version: snapshot
OS: 
Submission from: (NULL) (65.213.85.129)


There is really no good reason for throwing part of p's precision away in
pbinom.
Using Ambramowitz & Stegun 26.5.2:

-    return pbeta(1 - p, n - x, x + 1, lower_tail, log_p);
+    return pbeta (p, x + 1, n - x, !lower_tail, log_p);