LISTEN_PORT setting not configured by env variable shouldn't be forced

[Robonau]

for instance i would like to be able to change the listen port setting via the jsonrpc api but how it is currently setup i cant.
even if LISTEN_PORT is unset it uses the default value of 6888.
if i send the jsonrpc to change the listen port setting to say 6889 it will respond with success but not actually change it.

[YujiaCheng1996]

Agreed！Due to massive use of NAT by ISP, STUN needs LISTEN_PORT and DHT_LISTEN_PORT to be different!