Random I1 and MTU on install

I1 becomes a DNS query to a random popular domain (A/AAAA) instead of the
shared icloud.com response every key carried. The format sticks to tags both
the kernel module and amneziawg-go accept and is checked before awg1.conf is
written: a bad I1 fails the whole awg-quick up. With KEEP_DATA=y the old I1
is kept, or generated if missing — the server never checks it, old keys keep
their own.

MTU is picked from 1320–1376 (AWG_MTU overrides) and kept on reinstall.
awg-ctrl now reads MTU from awg1.conf for vpn:// keys instead of a constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013bNGYiDst62wH3ZGTRPFLJ

internal/ctrl/server.go

@@ -150,7 +150,7 @@ func New() (*Server, error) {
 	module, tools := readVersions()
 	s.c = Config{
 		ServerIP: ip, ServerPort: sp, ServerName: name,
-		DNS1: "1.1.1.1", DNS2: "1.0.0.1", MTU: 1376, Subnet: "10.9",
+		DNS1: "1.1.1.1", DNS2: "1.0.0.1", MTU: ReadMTU(s.confPath()), Subnet: "10.9",
 		Keepalive: keepaliveByGen[gen], Params: params, Gen: gen,
 		AwgModule: module, AwgTools: tools,
 	}