journal: install sigbus handler for journal tools too

This makes them robust regarding truncation. Ideally, we'd export this
as an API, but given how messy SIGBUS handling is, and the uncertain
ownership logic of signal handlers we should not do this (unless libc
one day invents a scheme how to sanely install SIGBUS handlers for
specific memory areas only). However, for now we can still make all our
own tools robust.

Note that external tools will only have read-access to the journal
anyway, where SIGBUS is much more unlikely, given that only writes are
subject to disk full problems.