BUG/MINOR: vars: fix set-var/unset-var exclusivity in the keyword parser

The parser checks first for "set-var" then "unset-var" from the updated
offset instead of testing it only when the other one fails, so it
validates this rule as "unset-var":

    http-request set-varunset-var(proc.a)

This should be backported everywhere relevant, though it's mostly harmless
as it's unlikely that some users are purposely writing this in their conf!

(cherry picked from commit 2819210a8328ff60505e499acf9f8bc323bec840)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/vars.c b/src/vars.c
index 5b35aa3..d533b18 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -736,11 +736,11 @@
 	const char *kw_name;
 	int flags, set_var = 0;
 
-	if (!strncmp(var_name, "set-var", 7)) {
+	if (strncmp(var_name, "set-var", 7) == 0) {
 		var_name += 7;
 		set_var   = 1;
 	}
-	if (!strncmp(var_name, "unset-var", 9)) {
+	else if (strncmp(var_name, "unset-var", 9) == 0) {
 		var_name += 9;
 		set_var   = 0;
 	}