MINOR: cfgparse-listen: introduce proxy_parse_removed_kw() and use it

MINOR: cfgparse-listen: introduce proxy_parse_removed_kw() and use it

A significant number of cfgparse_listen()'s keywords are handled
specifically because they're obsolete and continue to deliver a detailed
indication about what to use instead. The most complicated ones are
"dispatch" and "monitor-net" which even suggest the exact line to use
in order to replace them.

Let's introduce proxy_parse_removed_kw() to handle such keywords and
their excentricities. The function is just a regular keyword parser,
so that the keywords it handles are now in the standard array and point
to it.

For now we only implement "monitor-net" and "dispatch". The output
messages remain exactly the same.