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.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 1fb6094..5107890 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -21898,6 +21898,10 @@
   | 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.                 |
   |                   |                                                     |
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 7be9755..18a9ff5 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -32,6 +32,7 @@
 #include <haproxy/stream.h>
 #include <haproxy/stream_interface.h>
 #include <haproxy/trace.h>
+#include <haproxy/version.h>
 
 
 /* FCGI Connection flags (32 bits) */
@@ -188,7 +189,8 @@
 #define FCGI_SP_PATH_TRANS     0x00002000
 #define FCGI_SP_CONT_LEN       0x00004000
 #define FCGI_SP_HTTPS          0x00008000
-#define FCGI_SP_MASK           0x0000FFFF
+#define FCGI_SP_SRV_SOFT       0x00010000
+#define FCGI_SP_MASK           0x0001FFFF
 #define FCGI_SP_URI_MASK       (FCGI_SP_SCRIPT_NAME|FCGI_SP_PATH_INFO|FCGI_SP_REQ_QS)
 
 /* FCGI parameters used when PARAMS record is sent */
@@ -206,6 +208,7 @@
 	struct ist rem_addr;
 	struct ist rem_port;
 	struct ist cont_len;
+	struct ist srv_soft;
 	int https;
 	struct buffer *p;
 };
@@ -1413,6 +1416,12 @@
 		}
 	}
 
+	if (!(params->mask & FCGI_SP_SRV_SOFT)) {
+		params->srv_soft = ist2(b_tail(params->p), 0);
+		chunk_appendf(params->p, "HAProxy %s", haproxy_version);
+		params->srv_soft.len = b_tail(params->p) - params->srv_soft.ptr;
+	}
+
   end:
 	return 1;
   error:
@@ -1502,6 +1511,10 @@
 			p.n = ist("HTTPS");
 			p.v = ist("on");
 			goto encode;
+		case FCGI_SP_SRV_SOFT:
+			p.n = ist("SERVER_SOFTWARE");
+			p.v = params->srv_soft;
+			goto encode;
 		default:
 			goto skip;
 	}
@@ -2002,6 +2015,8 @@
 						params.mask |= FCGI_SP_PATH_TRANS;
 					else if (isteq(p.n, ist("https")))
 						params.mask |= FCGI_SP_HTTPS;
+					else if (isteq(p.n, ist("server_software")))
+						params.mask |= FCGI_SP_SRV_SOFT;
 				}
 				else if (isteq(p.n, ist("content-length"))) {
 					p.n = ist("CONTENT_LENGTH");
@@ -2082,6 +2097,7 @@
 	    !fcgi_encode_default_param(fconn, fstrm, &params, &outbuf, FCGI_SP_SCRIPT_FILE) ||
 	    !fcgi_encode_default_param(fconn, fstrm, &params, &outbuf, FCGI_SP_PATH_TRANS)  ||
 	    !fcgi_encode_default_param(fconn, fstrm, &params, &outbuf, FCGI_SP_CONT_LEN)    ||
+	    !fcgi_encode_default_param(fconn, fstrm, &params, &outbuf, FCGI_SP_SRV_SOFT)    ||
 	    !fcgi_encode_default_param(fconn, fstrm, &params, &outbuf, FCGI_SP_HTTPS)) {
 		TRACE_ERROR("error encoding default params", FCGI_EV_TX_RECORD|FCGI_EV_STRM_ERR, fconn->conn, fstrm);
 		goto error;