Namespace default configuration file location

[dvzrv]

src/config.h.in:
Namespace the default configuration file location with the project's
name "umurmur", as in that location also the required certificates are
expected by default and using a namespaced location makes sense as to
not clutter the SYSCONFDIR (e.g. /etc/) with more files, as a namespace
is necessary anyways.

CMakeLists.txt:
Install the default configuration file to a "umurmur" namespaced
location.

Fixes #169

[dvzrv]

@fatbob313 can this be merged?
From a packaging perspective a namespaced location is the only thing that makes sense, as that way arbitrary configuration related files such as TLS certificates can be added (e.g. in the context of certbot deploy hooks) and made available specifically for the umurmur application while rendering them inaccessible to other applications and users.