added MTU and PersistentKeepalive support

[elieobeid7]

Adds two new install-time prompts:

• Default client MTU (default: 1280, 0 = automatic/unset)
• Default PersistentKeepalive (default: 25, 0 = disabled/unset)
• Stores these defaults in /etc/wireguard/params (no new storage file).
• When adding a new client, prompts again with the install-time values as defaults:
• Pressing Enter uses the install-time defaults
• Optional per-client override supported
• Generated client configs include MTU = ... and/or PersistentKeepalive = ... only when non-empty.

Why

• On some networks, WireGuard performance can stall due to fragmentation/PMTUD issues; allowing a safe MTU (e.g., 1280) to be set as a default fixes this.
• PersistentKeepalive improves reliability behind NAT/mobile networks.
• For example I got a new server from OVH, they implement DDOS filtering and the vpn connection is too unstable without MTU and PersistentKeepalive, same problem with Hetzner, I tried, and pretty much all cloud providers are the same, only Ramnode netherlands datacenter works great without those params, but they're changing their datacenter so I switched to OVH
• you first specify the MTU and the PersistentKeepalive on first install, so you don't have to type it for every client, but you can change it if you want to, just like ipv4 or ipv6