REORG: config: extract the proxy parser into cfgparse-listen.c

This was the largest function of the whole file, taking a rough second
to build alone. Let's move it to a distinct file along with a few
dependencies. Doing so saved about 2 seconds on the total build time.
diff --git a/Makefile b/Makefile
index e41c4ff..422a853 100644
--- a/Makefile
+++ b/Makefile
@@ -904,7 +904,7 @@
 endif
 
 OBJS = src/proto_http.o src/cfgparse.o src/cfgparse-global.o            \
-       src/server.o src/stream.o                                        \
+       src/cfgparse-listen.o src/server.o src/stream.o                  \
        src/flt_spoe.o src/stick_table.o src/stats.o src/mux_h2.o        \
        src/checks.o src/haproxy.o src/log.o src/dns.o src/peers.o       \
        src/standard.o src/sample.o src/cli.o src/stream_interface.o     \