bootchart: put the bootchart into the journal.

This bit of code is mostly stolen from coredump.c. We construct
a simple journal message and append the bootchart file in the
journal automatically.

You can extract the latest bootchart from the current boot with
something like:

$ journalctl -b MESSAGE_ID=9f26aa562cf440c2b16c773d0479b518 --field=BOOTCHART

which prints it to stdout.

None of the other logic is touched. The journal entry is created
even if bootchart was run manually, which is probably wrong.