Remove problems with fishd under Cygwin.

Yes, it's an awful hack, but IPC support (and fork support as well -
even FAQ mentions that, and suggests "restarting process" to solve the
problem (http://cygwin.com/faq/faq.html#faq.using.fixing-fork-failures),
but let's ignore that for now) is simply broken in Cygwin. Having to try
to do exactly same thing in Cygwin, just so perhaps it will work is a
completely normal thing (not). I love Windows.

fishd.cpp

@@ -529,6 +529,14 @@ static bool acquire_socket_lock(const std::string &sock_name, std::string *out_l
 */
 static int get_socket(void)
 {
+    // Cygwin has random problems involving sockets. When using Cygwin,
+    // allow 20 attempts at making socket correctly.
+#ifdef __CYGWIN__
+    int attempts = 0;
+repeat:
+    attempts += 1;
+#endif
+
     int s, len, doexit = 0;
     int exitcode = EXIT_FAILURE;
     struct sockaddr_un local;
@@ -599,6 +607,10 @@ unlock:
 
     if (doexit)
     {
+        // If Cygwin, only allow normal quit when made lots of attempts.
+#ifdef __CYGWIN__
+        if (exitcode && attempts < 20) goto repeat;
+#endif
         exit_without_destructors(exitcode);
     }