Merge <servicehelper> from included config file

<servicehelper> is not supported in the included config file. It's not
merged in merge_included().

There's clearly no reason it shouldn't be supported in the included config
file along with <user>, <type> and others. It's quite reasonable for
a client to override the default servicehelper, e. g. in system-local.conf.

https://bugs.freedesktop.org/show_bug.cgi?id=51560