MINOR: proxy/checks: Move parsing of external-check option in checks.c

Parsing of the proxy directive "option external-check" have been moved in checks.c.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 7b33eb0..ddc7ba2 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -2375,12 +2375,8 @@
 				goto out;
 		}
 		else if (!strcmp(args[1], "external-check")) {
-			/* excute an external command to check servers' health */
-			free(curproxy->check_req);
-			curproxy->check_req = NULL;
-			curproxy->options2 &= ~PR_O2_CHK_ANY;
-			curproxy->options2 |= PR_O2_EXT_CHK;
-			if (alertif_too_many_args_idx(0, 1, file, linenum, args, &err_code))
+			err_code |= proxy_parse_external_check_opt(args, 0, curproxy, &defproxy, file, linenum);
+			if (err_code & ERR_FATAL)
 				goto out;
 		}
 		else if (!strcmp(args[1], "forwardfor")) {