scripted-diff: net_processing: Use existing chainman

-BEGIN VERIFY SCRIPT-
sed -i -E \
    -e 's/g_chainman/m_chainman/g' \
    -e 's@([^:])(Chain(state|)Active)@\1::\2@g' \
    -e 's@::Chain(state|)Active\(\)@m_chainman.ActiveChain\1()@g' \
    -- src/net_processing.cpp
-END VERIFY SCRIPT-

Author: dongcarl