network: rename [DHCP] section to [DHCPv4]

To keep the backward compatibility broken by the previous commit.

test/fuzz/fuzz-network-parser/directives.network

@@ -66,6 +66,33 @@ UseTimezone=
 RouteTable=
 BlackList=
 SendRelease=
+MaxAttempts=
+[DHCPv4]
+UseDNS=
+UseDomains=
+UseRoutes=
+IAID=
+UserClass=
+UseNTP=
+UseMTU=
+UseDomainName=
+RouteMetric=
+SendHostname=
+Anonymize=
+VendorClassIdentifier=
+Hostname=
+DUIDType=
+UseHostname=
+CriticalConnection=
+DUIDRawData=
+RequestBroadcast=
+ClientIdentifier=
+ListenPort=
+UseTimezone=
+RouteTable=
+BlackList=
+SendRelease=
+MaxAttempts=
 [DHCPv6]
 UseNTP=
 UseDNS=
@@ -218,10 +245,3 @@ MaxLeaseTimeSec=
 DefaultLeaseTimeSec=
 EmitTimezone=
 DNS=
-MaxAttempts=
-[DHCPv4]
-UseHostname=
-UseMTU=
-UseDomainName=
-CriticalConnection=
-UseDNS=