cgo: disallow use of C.errno
cgo: disallow use of C.errno

It's incorrect, and it causes cryptic linker errors.

Fixes issue 1360.

R=r, r2
CC=golang-dev
http://codereview.appspot.com/3880041

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang