syscall: add TCIOFLUSH family of constants

syscall: add TCIOFLUSH family of constants

Fixes #6355.

zerrors_linux_{386,amd64,arm}.go were regenerated using mkerrors.sh but I opted to add the three TC.*FLUSH lines by hand to keep the diff smaller and avoid problems with the API checker.

I'll check freebsd and darwin, could I ask for help with net/open bsd.

R=mikioh.mikioh, jsing, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/13660043