net: separate unix pollster initialization from network file descriptor allocation

Unlike the existing net package own pollster, runtime-integrated
network pollster on BSD variants, actually kqueue, requires a socket
that has beed passed to syscall.Listen previously for a stream
listener.

This CL separates pollDesc.Init of Unix network pollster from newFD
to avoid any breakages in the transition from Unix network pollster
to runtime-integrated pollster. Upcoming CLs will rearrange the call
order of pollster and syscall functions like the following;

- For dialers that open active connections, pollDesc.Init will be
  called in between syscall.Bind and syscall.Connect.

- For stream listeners that open passive stream connections,
  pollDesc.Init will be called just after syscall.Listen.

- For datagram listeners that open datagram connections,
  pollDesc.Init will be called just after syscall.Bind.

This is in preparation for runtime-integrated network pollster for BSD
variants.

Update #5199

R=dvyukov, bradfitz
CC=golang-dev
https://golang.org/cl/12663043

src/pkg/net/fd_poll_unix.go

@@ -252,14 +252,23 @@ func (pd *pollDesc) Close() {
 }
 
 func (pd *pollDesc) Lock() {
+	if pd.pollServer == nil {
+		return
+	}
 	pd.pollServer.Lock()
 }
 
 func (pd *pollDesc) Unlock() {
+	if pd.pollServer == nil {
+		return
+	}
 	pd.pollServer.Unlock()
 }
 
 func (pd *pollDesc) Wakeup() {
+	if pd.pollServer == nil {
+		return
+	}
 	pd.pollServer.Wakeup()
 }
 
@@ -294,6 +303,9 @@ func (pd *pollDesc) WaitWrite() error {
 }
 
 func (pd *pollDesc) Evict() bool {
+	if pd.pollServer == nil {
+		return false
+	}
 	return pd.pollServer.Evict(pd)
 }