Clean up and simplify BN_mod_lshift{,_quick}().

Clean up and simplify BN_mod_lshift{,_quick}().

BN_mod_lshift() already has a BN_CTX available, make use of it rather than
calling BN_dup() and BN_free().

In BN_mod_lshift_quick(), BN_copy() already handles dst == src, so avoid
checking this before the call. The max_shift == 0 case can also be handled
without code duplication. And as with other *_quick() functions, use
BN_ucmp() and BN_usub() directly given the 0 <= a < m constraint.

ok tb@