syscall: Linux-only support for parent death signal

As discussed in this thread:

https://groups.google.com/group/golang-dev/browse_thread/thread/5b76b7700265a787

I've tried to come up with a solution that is minimally invasive for the platforms that don't support "parent death signal", without splitting up exec_unix.go.

See also: http://www.win.tue.nl/~aeb/linux/lk/lk-5.html#ss5.8

R=rsc, dave, borman, iant, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5487061