MINOR/BUG: mworker/cli: do not use the unix_bind prefix for the master CLI socket

If the configuration file contains a 'unix-bind prefix' directive, and
if we use the -S option and specify a UNIX socket path, the path of the
socket will be prepended with the value of the unix-bind prefix.

For instance, if we have 'unix-bind prefix /tmp/sockets/' and we use
'-S /tmp/master-socket' on the command line, we will get this error:

Starting proxy MASTER:
cannot bind UNIX socket (No such file or directory) [/tmp/sockets/tmp/master-socket]

So this patch adds an exception, and will ignore the unix-bind prefix
for the master CLI socket.

This patch can be backported as far as 1.9.

(cherry picked from commit 1b8dacc858d7cb43488fb2ffca053274c571da1d)
[wt: cli_fe was called stats_fe in 2.3 and older]
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/cli.c b/src/cli.c
index f629b0c..979a751 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -89,7 +89,7 @@
 
 extern const char *stat_status_codes[];
 
-static struct proxy *mworker_proxy; /* CLI proxy of the master */
+struct proxy *mworker_proxy; /* CLI proxy of the master */
 
 static char *cli_gen_usage_msg(struct appctx *appctx)
 {