readahead: avoid gcc warning about format

src/readahead/readahead-common.c:55:17: warning: format ‘%zu’ expects argument of type ‘size_t’, but argument 7 has type ‘__off64_t’ [-Wformat=]
                 log_debug("Not preloading file %s with size out of bounds %zu", fn, st->st_size);
                 ^
Add systemd-coredumpctl as an alias for coredumpctl

Should make the transition easier for exisiting users.
core/transaction: reindent and split very long lines
core/transaction: avoid misleading error message when unit not found

There's no point in telling the user to look at the logs when
an attempt to load the unit file failed with ENOENT.

https://bugzilla.redhat.com/show_bug.cgi?id=996133
core/snapshot: log info when snapshots are created and removed

Snapshot mechanism is not used very much, but snapshot creation/destruction
should be at least as verbose as normal unit starting/stopping.
README: add gobject-introspection