core,seccomp: fix logic to parse syscall filter in dbus-execute.c

core,seccomp: fix logic to parse syscall filter in dbus-execute.c

If multiple SystemCallFilter= settings, some of them are whitelist
and the others are blacklist, are sent to bus, then the parse
result was corrupted.
This fixes the parse logic, now it is the same as one used in
load-fragment.c