syscall: use unsafe.Pointer instead of uintptr in net syscalls

syscall: use unsafe.Pointer instead of uintptr in net syscalls

In particular: setsockopt, getsockopt, bind, connect.

There are probably more.

All platforms cross-compile with make.bash, and all.bash still
pases on linux/amd64.

Update #7169

R=rsc
CC=golang-codereviews
https://golang.org/cl/55410043