Add abstract namespace support for IPC sockets on Linux.

Converts an initial strudel or "at sign" (@) in the Unix socket path to
a NULL character ('\0') indicating that the socket uses the abstract
namespace instead of the filesystem namespace.  For instance, binding a
socket to 'ipc://@/tmp/tester' will not create a file associated with
the socket whereas binding to 'ipc:///tmp/tester' will create the file
/tmp/tester.  See issue 567 for more information.

AUTHORS

@@ -20,6 +20,7 @@ Ben Gray <ben@benjamg.com>
 Bernd Prager <bernd@prager.ws>
 Bernd Melchers <melchers@ZEDAT.FU-Berlin.DE>
 Bob Beaty <rbeaty@peak6.com>
+Brandon Carpenter <hashstat@yahoo.com>
 Brian Buchanan <bwb@holo.org>
 Brett Cameron <Brett.Cameron@hp.com>
 Burak Arslan <burak-github@arskom.com.tr>

src/ipc_address.cpp

@@ -54,6 +54,10 @@ int zmq::ipc_address_t::resolve (const char *path_)
 
     address.sun_family = AF_UNIX;
     strcpy (address.sun_path, path_);
+#if defined ZMQ_HAVE_LINUX
+    if (*path_ == '@')
+        *address.sun_path = '\0';
+#endif
     return 0;
 }
 
@@ -65,7 +69,15 @@ int zmq::ipc_address_t::to_string (std::string &addr_)
     }
 
     std::stringstream s;
+#if !defined ZMQ_HAVE_LINUX
     s << "ipc://" << address.sun_path;
+#else
+    s << "ipc://";
+    if (*address.sun_path)
+       s << address.sun_path;
+    else
+       s << "@" << address.sun_path + 1;
+#endif
     addr_ = s.str ();
     return 0;
 }