Fix fmpz_read

[fredrik-johansson]

Fixes #2570: fmpz_read was completely broken (tried to read from stdout instead of stdin).

This fixes the qfb profile programs. Before:

$ build/qfb/profile/p-factor_qfb
Enter number to be factored: Read failed
Aborted (core dumped)

After:

$ build/qfb/profile/p-factor_qfb
Enter number to be factored: 1000000000000000000000033
iters = 10, multipliers = 10
iters = 20, multipliers = 11
Factor: 1726503814391