BUG/MINOR: mux-fcgi: Expose SERVER_SOFTWARE parameter by default

As specified in the RFC3875 (section 4.1.17), this parameter must be set to
the name and version of the information server software making the CGI
request. Thus, it is now added to the default parameters defined by
HAProxy. It is set to the string "HAProxy $version".

This patch should fix the issue #1285 and must be backported as far as 2.2.

(cherry picked from commit 5cd0e528cf2bed6eb1f79582ef89cf1667337e46)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 57cbaef..378ef7e 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -21895,6 +21895,10 @@
   | SERVER_PROTOCOL   | Contains the request's protocol.                    |
   |                   |                                                     |
   +-------------------+-----------------------------------------------------+
+  | SERVER_SOFTWARE   | Contains the string "HAProxy" followed by the       |
+  |                   | current HAProxy version.                            |
+  |                   |                                                     |
+  +-------------------+-----------------------------------------------------+
   | HTTPS             | Set to a non-empty value ("on") if the script was   |
   |                   | queried through the HTTPS protocol.                 |
   |                   |                                                     |