fpm: listen backlog should default to -1 also on Linux

On linux -1 means system administrator choosen default or kernel
default, this varies between kernel versions or distributions
it used to be 128 now it is 4096.

Author: crrodriguez

sapi/fpm/fpm/fpm_sockets.h

@@ -14,7 +14,7 @@
 /*
   On FreeBSD and OpenBSD, backlog negative values are truncated to SOMAXCONN
 */
-#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__linux__)
 #define FPM_BACKLOG_DEFAULT -1
 #else
 #define FPM_BACKLOG_DEFAULT 511