Reset signal-mask on re-exec to init=..

Process 1 (aka init) needs to be started with an empty signal mask.
That includes the process 1 that's started after the initrd is finished.
When the initrd is using systemd (as it does with dracut based initrds)
then it is systemd that calls the real init.  Normally this is systemd
again, except when the user uses for instance "init=/bin/bash" on the
kernel command line.