Fixed milltask segfault on exit

I caused this in yesterday's changes to src/emc/task/emctaskmain.cc.
I neglected to clear the sa_flags field of the sigaction struct. One guess is the SA_SIGINFO bit was set causing a stack frame mismatch for the handler.

I also changed three additional "signal()" calls to "sigaction()" for SIGSEV, SIGFPE, and SIGUSR1.