[BUGFIX] Fix corner cases of factor_multivariate().

[BUGFIX] Fix corner cases of factor_multivariate().

In some rare cases, factor() failed with what looks like an endless
loop trying evaluation homomorphisms with ever increasing moduli (or
maybe it just took eons to complete).

Whether this happened depended on which symbol was left unevaluated
for univariate factorization. As a workaround for this problem, we now
apply the evaluation homomorphism for each symbol in a round-robin
mode until one of them succeeds.

This somewhat speeds up factorization of multivariate polynomials
even in those cases where the old implementation terminated in time.

Thanks to Vitaly Magerya <vmagerya@gmail.com> for reporting this.