We cannot safely log between fork() and exec() because it isn't an
async-signal-safe operation (in particular it might allocate memory).

We also don't want to treat a failure here as a real problem, because
it might legitimately not work: in a system dbus-daemon that has dropped
privileges from root, the pseudo-file representing this process parameter
remains owned by root and cannot be altered by the unprivileged user.

For the main use-case for this operation, the system dbus-daemon, we
have another opportunity to do this in the dbus-daemon-launch-helper
(see the previous commit).

Signed-off-by: Simon McVittie <smcv@collabora.com>