Do not disable seccomp when configuration is not found

Previously, when seccomp configuration file for a service was not
found, the service was started without seccomp. I consider this
potential attack vector.

With this change, procd starts the service as if the configuration
existed but the service fails in libpreload-seccomp.so, because the
configuration cannot be loaded. This is announced in the syslog.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>