unix/stream: provide uipc_cantrcvmore()

and use it the pr_shutdown method. While unix/dgram can still use generic
socket socantrcvmore(), the stream versions need a specific one. This fixes
a panic reported by syzkaller. While here inline unp_shutdown() into
uipc_shutdown().

Reported-by: syzbot+86c18f0886f70a3509c6@syzkaller.appspotmail.com