MINOR: startup: HAPROXY_STARTUP_VERSION contains the version used to start

HAPROXY_STARTUP_VERSION: contains the version used to start, in
master-worker mode this is the version which was used to start the
master, even after updating the binary and reloading.

This patch could be backported in every version since it is useful when
debugging.

doc/configuration.txt

@@ -778,6 +778,10 @@ file, or could be inherited by a program (See 3.7. Programs):
 * HAPROXY_MASTER_CLI: In master-worker mode, listeners addresses of the master
   CLI, separated by semicolons.
 
+* HAPROXY_STARTUP_VERSION: contains the version used to start, in master-worker
+  mode this is the version which was used to start the master, even after
+  updating the binary and reloading.
+
 In addition, some pseudo-variables are internally resolved and may be used as
 regular variables. Pseudo-variables always start with a dot ('.'), and are the
 only ones where the dot is permitted. The current list of pseudo-variables is:

src/haproxy.c

@@ -1490,6 +1490,8 @@ static void init_early(int argc, char **argv)
 	char *tmp;
 	int len;
 
+	setenv("HAPROXY_STARTUP_VERSION", HAPROXY_VERSION, 0);
+
 	/* First, let's initialize most global variables */
 	totalconn = actconn = listeners = stopping = 0;
 	killed = pid = 0;