coredump: dump priviliges when processing system coredumps Let's add an extra-safety net and change UID/GID to the "systemd-coredump" user when processing coredumps from system user. For coredumps of normal users we keep the current logic of processing the coredumps from the user id the coredump was created under. Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=87354

turn kdbus support into a runtime option ./configure --enable/disable-kdbus can be used to set the default behavior regarding kdbus. If no kdbus kernel support is available, dbus-dameon will be used. With --enable-kdbus, the kernel command line option "kdbus=0" can be used to disable kdbus. With --disable-kdbus, the kernel command line option "kdbus=1" is required to enable kdbus support.

build-sys: configure the list of system users, files and directories Choose which system users defined in sysusers.d/systemd.conf and files or directories in tmpfiles.d/systemd.conf, should be provided depending on comile-time configuration.