abstract Unix-domain sockets

During the discussion on Unix-domain sockets on Windows, someone pointed
out[0] abstract Unix-domain sockets. This is a variant of the normal
Unix-domain sockets that don't use the file system but a separate
"abstract" namespace. At the user interface, such sockets are
represented by names starting with "@". I took a look at this and it
wasn't hard to get working, so here is a patch. It's supposed to be
supported on Linux and Windows right now, but I haven't tested on Windows.

I figure, there are so many different deployment options nowadays, this
could be useful somewhere. It relieves you from dealing with the file
system, you don't have to set up /tmp or something under /var/run, you
don't need to make sure file system permissions are right. Also, there
is no need for a lock file or file cleanup. (Unlike file-system
namespace sockets, abstract namespace sockets give an EADDRINUSE when
trying to bind to a name already in use.) Conversely, of course, you
don't get to use file-system permissions to manage access to the socket,
but that isn't essential functionality, so it's a trade-off users can
make on their own.

And then some extra patches for surrounding cleanup. During testing I
noticed that the bind() failure hint "Is another postmaster already
running ..." was shown in inappropriate situations, so I changed that to
only show for EADDRINUSE errors. (Maybe other error codes could be
appropriate, but I couldn't find any more.)

And then looking for other uses of EADDRINUSE I found some dead
Windows-related code that can be cleaned up.

[0]:
https://www.postgresql.org/message-id/20191218142419.fvv4ikm4wq4gnkco@isc.upenn.edu

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services