tmpfiles: in C lines, make missing source graceful error

tmpfiles: in C lines, make missing source graceful error

I don't see where it would ever be a good thing that file copies done
via tmpfiles.d/ C lines cause the tmpfiles operation to fail if their
source happens to be missing. It's a problem if we can't set up the
destination properly (which is the job of systemd-tmpfiles after all),
but if the source is simply missing (NB: setting up the source is the job of
of the rules writer) this shouldn't be a problem.

This is useful for copying stuff into place if it happens to exist. For
example, if systemd-stub passes additional data into the initrd's
/.extra/ directory, we can copy it into a better place (e.g. /run/) with
this, where it will survive the initrd→host transition.

This mirrors behaviour of the recently added "^" line modifier which may
be used source "w" lines from credentials – there two the behaviour is
to simply skip the line if the source is missing.