add implementations for llge_prev, llget_next, llget_last, llget_first. fixed not setting list->current in several functions.
add implementations for llge_prev, llget_next, llget_last, llget_first.
fixed not setting list->current in several functions.

zcutlip/basic-data-structures,zcutlip/basic-data-structures,zcutlip/basic-data-structures