Merge <servicehelper> from included config file

Merge <servicehelper> from included config file

<servicehelper> is not supported in the included config file, i. e. 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 willing to override the
default servicehelper, e. g. in system-local.conf.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=51560
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>