service,properties: add support for leftmodecfgclient
Previously the plugin always set leftmodecfgclient=yes
, which is used
for roaming clients to obtain a dynamic IP. In a server-to-server
scenario we don't want that option; allow omitting it by passing
leftmodecfgclient=no
. It's somehow confusing that the new option has
the opposite default value than Libreswan, but that's the only way to
keep backwards compatibility for existing configurations.