ipconfig: Support complete disabling of IPv6 and refactor

[ipconfig] Support complete disabling of IPv6 and refactor. JB#48769

Support complete disabling of IPv6 on both system and interface levels.
Disabling of IPv6 completely albeit temporarily is required when there
can be multiple connected technologies and a VPN is connected over IPv4
using an dual-stack transport and/or there is another technology with
dual-stack or only IPv6 in use. This approach effectively ensures that
no data from an IPv4 VPN leaks to any other interface. A DNS server
returning replies to AAAA requests may return an IPv6 address, which
could be then used over IPv6 routing on another technolology that is not
acting as the transport of the VPN.

Add a "ipv6_force_disabled" toggle to indicate that the particular
device tied to ipconfig should not have IPv6 enabled. This toggle is to
be used in scenarios where IPv6 should be prevented from reinstating
unless forcefully enabled. The __connman_ipconfig_{enable,disable}_ipv6()
now have additional boolean to control this behavior, and the enabling
function returns either -EINVAL or -EOPNOTSUPP for errors and 0 when
success.

When checking IPv6 enabled state return false if ipv6_force_disabled is
set as that is the real status of IPv6. The /proc disable_ipv6 can
change when kernel processes ICMPv6 (RA/RS) packets and brings the IPv6
interface up so using only the disable_ipv6 value is not consistent
enough in case when IPv6 has been forcefully disabled.

This change will prevent from changing the IPv6 status until it has
it has been forcefully enabled. The most prominent use case for this
is to prevent data as well as DNS leak to IPv6 when IPv4 only VPN is
connected over a transport supporting both IPv4 and IPv6 connectivity.

In addition to changing disable_ipv6 value also the autoconf option is
managed. This is done in order to control the address setup for the
interface. Also accept_ra is managed to prevent from setting up the
addressing according to route advertizements. The accept_ra value is
backed up before disabling in order to restore it to the correct value
when re-enabling. It is worthy to note that disable_ipv6 may be
temporarily set to 0 by the kernel when RS packets are received and LL
addess is temporarily set as a typical behavior of IPv6 on some
configurations, and after a while the value of disable_ipv6 as well as
LL address are reset/removed by kernel.

Also refactor getting and setting the proc conf values, have read and
write in their own respective general functions. If write/read is
issued without interface name (NULL) then "all" section is used to
follow the old behavior.

connman/src/connman.h

@@ -435,8 +435,11 @@ void __connman_ipconfig_set_ops(struct connman_ipconfig *ipconfig,
 				const struct connman_ipconfig_ops *ops);
 int __connman_ipconfig_set_method(struct connman_ipconfig *ipconfig,
 					enum connman_ipconfig_method method);
-void __connman_ipconfig_disable_ipv6(struct connman_ipconfig *ipconfig);
-void __connman_ipconfig_enable_ipv6(struct connman_ipconfig *ipconfig);
+void __connman_ipconfig_disable_ipv6(struct connman_ipconfig *ipconfig,
+								bool forced);
+int __connman_ipconfig_enable_ipv6(struct connman_ipconfig *ipconfig,
+								bool forced);
+int __connman_ipconfig_set_ipv6_support(bool enable);
 
 int __connman_ipconfig_init(void);
 void __connman_ipconfig_cleanup(void);