Update #1435

This proposal disables Setuid and Setgid on all linux platforms.

Issue 1435 has been open for a long time, and it is unlikely to be addressed soon so an argument was made by a commenter

https://code.google.com/p/go/issues/detail?id=1435#c45

That these functions should made to fail rather than succeed in their broken state.

LGTM=ruiu, iant
R=iant, ruiu
CC=golang-codereviews
https://golang.org/cl/106170043