Specifying required_dirs and creating it at prestart stage does not work
because required_dirs is checked before running prestart these days.
So it fails to start for mfs-based /var even if racoon_create_dirs=YES
Unbreak this by replacing "required_dirs" and "mkdir -p"
in the racoon_prestart with "install -d" that returns error in case
of failure and does nothing if the directory already exists.
Reported by: Cybil Courraud <freebsd@cyb.fr>
ad7221c383