net_utils: move is_ipv4_literal into net_utils
Merge pull request #535 from msimerson/net_utils

net_utils: move is_ipv4_literal into net_utils
qmd: when relaying, validate MAIL FROM against QMD

and refactored the code a little, to avoid redundancy
qmd: added config:check_outbound option
qmd: move the callback to the last argument

to get_qmd_response