man: delete ZX as sole compression; "Compress=" as boolean

In man journald.conf, removes reference to XZ as sole form of
compression. See commit d89c8fdf48c7bad5816b9f2e77e8361721f22517.

In man coredump.conf, clarifies that "Compression=" controls existence,
not type, of compression.
man: BusName= is not optional for Type!=bus

http://lists.freedesktop.org/archives/systemd-devel/2014-November/025492.html
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.