fix: disable ifconfig-pool-persist when duplicate-cn is enabled (#1354)

## Summary
- Only add `ifconfig-pool-persist` to server.conf when `MULTI_CLIENT !=
y`
- Add note in the installation prompt about this limitation
- Update README to mention the trade-off

First reported in
https://github.com/angristan/openvpn-install/issues/440#issuecomment-2987417197

## Background

`ifconfig-pool-persist` is incompatible with `duplicate-cn`. When
`duplicate-cn` is enabled, OpenVPN bypasses common name matching in the
IP pool allocation, making the persistence file ineffective.

From [OpenVPN
source](https://github.com/OpenVPN/openvpn/blob/e5ff8247/src/openvpn/init.c#L3608-L3610):
```c
if (o->duplicate_cn && o->ifconfig_pool_persist_filename)
{
    msg(M_WARN, "WARNING: --ifconfig-pool-persist will not work with --duplicate-cn");
}
```

Previously, the script always added `ifconfig-pool-persist ipp.txt`
regardless of whether `duplicate-cn` was enabled via `MULTI_CLIENT=y`.

README.md

@@ -140,7 +140,7 @@ export PASS="1"
 - Block DNS leaks on Windows 10
 - Randomised server certificate name
 - Choice to protect clients with a password (private key encryption)
-- Option to allow multiple devices to use the same client profile simultaneously
+- Option to allow multiple devices to use the same client profile simultaneously (disables persistent IP addresses)
 - Many other little things!
 
 ## Compatibility