This is a thing that was in the python config library, but COMMON
serves the same purpose.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Keys are looked up in that order, unless otherwise specified.
<client> is the client's virtual address.
<servername> must be a valid lowercase DNS hostname and not look like
Keys are looked up in that order, unless otherwise specified.
<client> is the client's virtual address.
<servername> must be a valid lowercase DNS hostname and not look like
-an address, or be COMMON, DEFAULT or SERVER.
+an address, or be COMMON or SERVER.