MINOR: mux-fcgi: Make the capture of the path-info optional in pathinfo regex

MINOR: mux-fcgi: Make the capture of the path-info optional in pathinfo regex

Now, only one capture is mandatory in the path-info regex, the one matching the
script-name. The path-info capture is optional. Of couse, it must be defined to
fill the PATH_INFO parameter. But it is not mandatory. This way, it is possible
to get the script-name part from the path, excluding the path-info.

This patch is small enough to be backported to 2.1.

(cherry picked from commit 6c57f2da4326768c08603480f0ee2fa0a7f73231)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>