add difficultyadjinterval to the same position in getblockchaininfo as on MVF-BU, add help and refactor a little
add difficultyadjinterval to the same position in getblockchaininfo as on MVF-BU, add help and refactor a little

the help + refactoring should be ported back to MVF-BU
BTCfork/hardfork_prototype_1_mvf-core,BTCfork/hardfork_prototype_1_mvf-core,BTCfork/hardfork_prototype_1_mvf-core,BTCfork/hardfork_prototype_1_mvf-core,BTCfork/hardfork_prototype_1_mvf-core,BTCfork/hardfork_prototype_1_mvf-core