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/fcgi-app.c b/src/fcgi-app.c
index 71f6dd4..4a7e0b3 100644
--- a/src/fcgi-app.c
+++ b/src/fcgi-app.c
@@ -581,7 +581,7 @@
 
 /* Parses the "use-fcgi-app" proxy keyword */
 static int proxy_parse_use_fcgi_app(char **args, int section, struct proxy *curpx,
-				    struct proxy *defpx, const char *file, int line,
+				    const struct proxy *defpx, const char *file, int line,
 				    char **err)
 {
 	struct flt_conf *fconf = NULL;