ssh-proxy: use % as an alternative separator

Since OpenSSH 10.0p1, specifically
https://anongit.mindrot.org/openssh.git/commit/?id=487cf4c18c123b66c1f3f733398cd37e6b2ab6ab
ssh refuses comma in hostname by default.

Let's use % as an alternative separator.

Follow-up for defd5060d6.

src/ssh-generator/ssh-proxy.c

@@ -178,7 +178,7 @@ static int process_machine(const char *machine, const char *port) {
 static char *startswith_sep(const char *s, const char *prefix) {
         const char *p = startswith(s, prefix);
 
-        if (p && IN_SET(*p, '/', ','))
+        if (p && IN_SET(*p, '/', '%'))
                 return (char*) p + 1;
 
         return NULL;