MINOR: global: keep a copy of the initial rlim_fd_cur and rlim_fd_max values

Let's keep a copy of these initial values. They will be useful to
compute automatic maxconn, as well as to restore proper limits when
doing an execve() on external checks.
diff --git a/src/haproxy.c b/src/haproxy.c
index 59da1fd..62263e4 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -220,6 +220,8 @@
 struct list proc_list = LIST_HEAD_INIT(proc_list);
 
 int master = 0; /* 1 if in master, 0 if in child */
+unsigned int rlim_fd_cur_at_boot = 0;
+unsigned int rlim_fd_max_at_boot = 0;
 
 struct mworker_proc *proc_self = NULL;
 
@@ -2748,6 +2750,11 @@
 		exit(1);
 	}
 
+	/* take a copy of initial limits before we possibly change them */
+	getrlimit(RLIMIT_NOFILE, &limit);
+	rlim_fd_cur_at_boot = limit.rlim_cur;
+	rlim_fd_max_at_boot = limit.rlim_max;
+
 	/* process all initcalls in order of potential dependency */
 	RUN_INITCALLS(STG_PREPARE);
 	RUN_INITCALLS(STG_LOCK);