Use default ULA prefix if fixed-cidr-v6 is not specified

Use the same logic to generate IPAMConf for IPv6 as for IPv4.

- When no fixed-cidr-v6 is specified, rather than error out, use
  the default address pools (as for an IPv4 default bridge with no
  fixed-cidr, and as for user-defined networks).
- Add daemon option --bip6, similar to --bip.
  - Necessary because it's the only way to override an old address
    on docker0 (daemon-managed default bridge), as illustrated by
    test cases.
- For a user-managed default bridge (--bridge), use IPv6 addresses
  on the user's bridge to determine the pool, sub-pool and gateway.
  Following the same rules as IPv4.
- Don't set up IPv6 IPAMConf if IPv6 is not enabled.

Signed-off-by: Rob Murray <rob.murray@docker.com>

man/dockerd.8.md

@@ -10,6 +10,7 @@ dockerd - Enable daemon mode
 [**--authorization-plugin**[=*[]*]]
 [**-b**|**--bridge**[=*BRIDGE*]]
 [**--bip**[=*BIP*]]
+[**--bip6**[=*BIP*]]
 [**--cgroup-parent**[=*[]*]]
 [**--config-file**[=*path*]]
 [**--containerd**[=*SOCKET-PATH*]]
@@ -146,7 +147,11 @@ $ sudo dockerd --add-runtime runc=runc --add-runtime custom=/usr/local/bin/my-ru
   container networking
 
 **--bip**=""
-  Use the provided CIDR notation address for the dynamically created bridge
+  Use the provided CIDR notation IPv4 address for the dynamically created bridge
+  (docker0); Mutually exclusive of \-b
+
+**--bip6**=""
+  Use the provided CIDR notation IPv6 address for the dynamically created bridge
   (docker0); Mutually exclusive of \-b
 
 **--cgroup-parent**=""