avutil/opt: remove dead code from set_string_number()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil: move QP2LAMBDA constant to eval

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/opt: Support general expressions involving AVOption constants in set_string_number()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>