CLEANUP: config: make the cfg_keyword parsers take a const for the defproxy

The default proxy was passed as a variable to all parsers instead of a
const, which is not without risk, especially when some timeout parsers used
to make some int pointers point to the default values for comparisons. We
want to be certain that none of these parsers will modify the defaults
sections by accident, so it's important to mark this proxy as const.

This patch touches all occurrences found (89).
diff --git a/src/connection.c b/src/connection.c
index b1d643b..ac098d3 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -1298,7 +1298,7 @@
 
 /* returns 0 on success */
 static int cfg_parse_pp2_never_send_local(char **args, int section_type, struct proxy *curpx,
-                                          struct proxy *defpx, const char *file, int line,
+                                          const struct proxy *defpx, const char *file, int line,
                                           char **err)
 {
 	if (too_many_args(0, args, err, NULL))