package/audit/S02auditd: fix shellcheck and check-package warnings

Fixup S20audit to pass shellcheck -oall and check-package. The file now
closely resembles package/busybox/S01syslogd.

Tested with qemu_x86_64_defconfig. start, stop, restart, reload, and rotate
all work with busybox ash shell.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
[Arnout: remove it from .checkpackageignore]
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>