MEDIUM: init: set default for fd_hard_limit via DEFAULT_MAXFD

Let's provide a default value for fd_hard_limit, if it's not set in the
configuration. With this patch we could set some specific default via
compile-time variable DEFAULT_MAXFD as well. Hope, this will be helpfull for
haproxy package maintainers.

    make -j 8 TARGET=linux-glibc DEBUG=-DDEFAULT_MAXFD=50000

If haproxy is comipled without DEFAULT_MAXFD defined, the default will be set
to 1048576.

This is done to avoid killing the process by its watchdog, while it started
without any limitations in its configuration or in the command line and the
hard RLIMIT_NOFILE is extremely huge (~1000000000). We use in this case
compute_ideal_maxconn() to calculate maxconn and maxsock, maxsock defines the
size of internal fdtab, which becames very-very large as well. When
the process starts to simply loop over this fdtab (0(n)), this takes a lot of
time, so watchdog does it job.

To avoid this, maxconn now is always reduced to some reasonable value either
by explicit global.fd-hard-limit from configuration, or by its default. The
default may be changed at build-time and overwritten then by
global.fd-hard-limit at runtime. Explicit global.fd-hard-limit from the
configuration has always precedence over DEFAULT_MAXFD, if set.

Must be backported in all stable versions until v2.6.0, including v2.6.0.

include/haproxy/defaults.h

@@ -295,6 +295,24 @@
 #define DEFAULT_MAXCONN 100
 #endif
 
+/* Default file descriptor limit.
+ *
+ * DEFAULT_MAXFD explicitly reduces the hard RLIMIT_NOFILE, which is used by the
+ * process as the base value to calculate the default global.maxsock, if
+ * global.maxconn, global.rlimit_memmax are not defined. This is useful in the
+ * case, when hard nofile limit has been bumped to fs.nr_open (kernel max),
+ * which is extremely large on many modern distros. So, we will also finish with
+ * an extremely large default global.maxsock. The only way to override
+ * DEFAULT_MAXFD, if defined, is to set fd_hard_limit in the config global
+ * section. If DEFAULT_MAXFD is not set, a reasonable maximum of 1048576 will be
+ * used as the default value, which almost guarantees that a process will
+ * correctly start in any situation and will be not killed then by watchdog,
+ * when it will loop over the allocated fdtab.
+*/
+#ifndef DEFAULT_MAXFD
+#define DEFAULT_MAXFD 1048576
+#endif
+
 /* Define a maxconn which will be used in the master process once it re-exec to
  * the MODE_MWORKER_WAIT and won't change when SYSTEM_MAXCONN is set.
  *