CP-12909: Support a directory full of configuration file fragments
CP-12909: Support a directory full of configuration file fragments

The main service config file can contain sensible defaults, while
site-specific overrides can be placed in a fragment file. This also
allows packages to modify the configuration of another package,
without worry that the changes will be lost over upgrade.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

xapi-project/message-switch,xapi-project/message-switch,euanh/xcp-idl,johnelse/xcp-idl,Chunjie/xcp-idl,djs55/xcp-idl,xapi-project/message-switch