strongswan: check return value instead of output

strongswan: check return value instead of output

When the strongswan service is running, `ipsec status` returns 0. Check
the return value instead of checking its output.

While at it, remove the [[ ]] bashism, use rereadall instead of
(reread)secrets, and move it inside the if statement.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>