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
diff --git a/src/pkg/syscall/syscall_linux_amd64.go b/src/pkg/syscall/syscall_linux_amd64.go
index f4b73b2..5559590 100644
--- a/src/pkg/syscall/syscall_linux_amd64.go
+++ b/src/pkg/syscall/syscall_linux_amd64.go
@@ -40,18 +40,18 @@
 //sys	Truncate(path string, length int64) (err error)
 //sys	accept(s int, rsa *RawSockaddrAny, addrlen *_Socklen) (fd int, err error)
 //sys	accept4(s int, rsa *RawSockaddrAny, addrlen *_Socklen, flags int) (fd int, err error)
-//sys	bind(s int, addr uintptr, addrlen _Socklen) (err error)
-//sys	connect(s int, addr uintptr, addrlen _Socklen) (err error)
+//sys	bind(s int, addr unsafe.Pointer, addrlen _Socklen) (err error)
+//sys	connect(s int, addr unsafe.Pointer, addrlen _Socklen) (err error)
 //sysnb	getgroups(n int, list *_Gid_t) (nn int, err error)
 //sysnb	setgroups(n int, list *_Gid_t) (err error)
-//sys	getsockopt(s int, level int, name int, val uintptr, vallen *_Socklen) (err error)
-//sys	setsockopt(s int, level int, name int, val uintptr, vallen uintptr) (err error)
+//sys	getsockopt(s int, level int, name int, val unsafe.Pointer, vallen *_Socklen) (err error)
+//sys	setsockopt(s int, level int, name int, val unsafe.Pointer, vallen uintptr) (err error)
 //sysnb	socket(domain int, typ int, proto int) (fd int, err error)
 //sysnb	socketpair(domain int, typ int, proto int, fd *[2]int32) (err error)
 //sysnb	getpeername(fd int, rsa *RawSockaddrAny, addrlen *_Socklen) (err error)
 //sysnb	getsockname(fd int, rsa *RawSockaddrAny, addrlen *_Socklen) (err error)
 //sys	recvfrom(fd int, p []byte, flags int, from *RawSockaddrAny, fromlen *_Socklen) (n int, err error)
-//sys	sendto(s int, buf []byte, flags int, to uintptr, addrlen _Socklen) (err error)
+//sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)