Linux multiprocess crash reporter: use pipe instead of socketpair in the client.

This makes the Linux multiprocess crash generation client easier to sandbox with
seccomp-bpf when chrooting and namespaces can't be used: if sendmsg and
socketpair are both allowed, then the process can send to arbitrary Unix-domain
named datagram sockets, which is potentially bad.  The filter can't inspect
socket-related calls' arguments on 32-bit x86 (see the socketcall(2) man page),
which includes the "type" parameter to socketpair(2).

Thus, this patch replaces the socket pair with a pipe.  This depends on issue
7724002 (although it's possible to adjust the code in question rather than
deleting it, if need be).

The other socket dependency is that the server sends a single byte to the child
with the MSG_DONTWAIT | MSG_NOSIGNAL flags — but the child ignores whether or
not that byte was read, so it suffices to simply close the fd and let the child
encounter end-of-file.  This patch does that.

[Ted Mielczarek]

LGTM