MEDIUM: fcgi-app: Add FCGI application and filter

The FCGI application handles all the configuration parameters used to format
requests sent to an application. The configuration of an application is grouped
in a dedicated section (fcgi-app <name>) and referenced in a backend to be used
(use-fcgi-app <name>). To be valid, a FCGI application must at least define a
document root. But it is also possible to set the default index, a regex to
split the script name and the path-info from the request URI, parameters to set
or unset...  In addition, this patch also adds a FCGI filter, responsible for
all processing on a stream.
diff --git a/Makefile b/Makefile
index 0d760b7..5996d80 100644
--- a/Makefile
+++ b/Makefile
@@ -787,7 +787,7 @@
        src/protocol.o src/arg.o src/hpack-huff.o src/base64.o src/ring.o      \
        src/hash.o src/mailers.o src/activity.o src/version.o src/trace.o      \
        src/mworker.o src/mworker-prog.o src/debug.o src/wdt.o src/dict.o      \
-       src/xprt_handshake.o src/h1_htx.o src/fcgi.o
+       src/xprt_handshake.o src/h1_htx.o src/fcgi.o src/fcgi-app.o
 
 EBTREE_OBJS = $(EBTREE_DIR)/ebtree.o $(EBTREE_DIR)/eb32sctree.o \
               $(EBTREE_DIR)/eb32tree.o $(EBTREE_DIR)/eb64tree.o \