Similarly to what systemd-resolved does, introduce the concept of
"routing" domain, which is a domain in the search list that is used
only to decide the interface over which a query must be forwarded, but
is not used to complete unqualified host names. Routing domains are
those starting with a tilde ('~') before the actual domain name.
Domains without the initial tilde are used both for completing
unqualified names and for the routing decision.
The `settings-docs.c` file is generated by processing the
`nm-property-docs.xml` file. Although this works in autotools,
the `.c` extension makes meson not to handle it properly.
Given the fact that it only contains a number of defines it
makes sense to change its extension to `.h` an use it as a header.
This also makes meson to handle it properly and build it before
its used.
https://mail.gnome.org/archives/networkmanager-list/2018-January/msg00057.html
