Fixes: b42f780e045c69e6b2c350dcd6de760c673a78e0
Fixes: d252a99fa25359e3795abcacfae08c76dc5386b8

It causes the links to be based on a latest online version rather than
on the current one.

As with NMVpnPluginOld, rename NMSecretAgent to NMSecretAgentOld, to
leave the name "NMSecretAgent" open for a new-and-improved version in
NM 1.2.

Update the docs build to include and exclude the correct files.

Fill in some missing documentation, and fix problems in the existing
docs. (In particular, "<" can't appear as a literal in documentation,
so change it to "&lt;". Also, "PKCS#12" has to be written as
"PKCS#<!-- -->12", or gtk-doc will think "#12" is a reference to a
type named "12".)

As with NMIP4Config and NMIP6Config, merge the two DHCP config classes
into one in the public API.

Create NMIPConfig as the parent of NMIP4Config and NMIP6Config, and
remove the two subclasses from the public API; while it's convenient
to still have both internally, they are now identical to the outside
world.

Merge NMIP4Address and NMIP6Address into NMIPAddress, and NMIP4Route
and NMIP6Route into NMIPRoute. The new types represent IP addresses as
strings, rather than in binary, and so are address-family agnostic.