[ipv6] Rename ipv6_scope to dhcpv6_scope

The settings scope ipv6_scope refers specifically to IPv6 settings
that have a corresponding DHCPv6 option.  Rename to dhcpv6_scope to
more accurately reflect this purpose.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

 };
 
 /** IPv6 settings scope */
-const struct settings_scope ipv6_scope;
+const struct settings_scope dhcpv6_scope;
 
 /**
  * Integer setting type indices

src/include/ipxe/settings.h

 extern const struct settings_scope builtin_scope;
 
 /** IPv6 setting scope */
-extern const struct settings_scope ipv6_scope;
+extern const struct settings_scope dhcpv6_scope;
 
 /**
  * A generic settings block

src/net/udp/dhcpv6.c

 static int dhcpv6_applies ( struct settings *settings __unused,
 			    const struct setting *setting ) {
 
-	return ( setting->scope == &ipv6_scope );
+	return ( setting->scope == &dhcpv6_scope );
 }
 
 /**
 	}
 	ref_init ( &dhcpv6set->refcnt, NULL );
 	settings_init ( &dhcpv6set->settings, &dhcpv6_settings_operations,
-			&dhcpv6set->refcnt, &ipv6_scope );
+			&dhcpv6set->refcnt, &dhcpv6_scope );
 	data = ( ( ( void * ) dhcpv6set ) + sizeof ( *dhcpv6set ) );
 	len = options->len;
 	memcpy ( data, options->data, len );
 	.description = "Boot filename",
 	.tag = DHCPV6_BOOTFILE_URL,
 	.type = &setting_type_string,
-	.scope = &ipv6_scope,
+	.scope = &dhcpv6_scope,
 };
 
 /** DNS search list setting */
 	.description = "DNS search list",
 	.tag = DHCPV6_DOMAIN_LIST,
 	.type = &setting_type_dnssl,
-	.scope = &ipv6_scope,
+	.scope = &dhcpv6_scope,
 };

src/net/udp/dns.c

 	.description = "DNS server",
 	.tag = DHCPV6_DNS_SERVERS,
 	.type = &setting_type_ipv6,
-	.scope = &ipv6_scope,
+	.scope = &dhcpv6_scope,
 };
 
 /** DNS search list */

src/net/udp/syslog.c

 	.description = "Syslog server",
 	.tag = DHCPV6_LOG_SERVERS,
 	.type = &setting_type_ipv6,
-	.scope = &ipv6_scope,
+	.scope = &dhcpv6_scope,
 };
 
 /**