Install config to /etc/umurmur.conf

[dvzrv]

CMakeLists.txt:
The changes introduced in #164 install the configuration to
/etc/umurmur/umurmur.conf by default. However, umurmurd is picking up
the configuration from /etc/umurmur.conf automatically, which makes that
location a more logical default.

[anthraxx]

Or, depending what upstream wishes, the location in the code can be extended to also support /etc/umurmur/umurmur.conf. I think directory namespaces in /etc are not a bad idea overall, they are naturally namespacing a group of configs if ever more files would be added.

[fatbob313]

I ran into some problems with config file location which was solved by using CMAKE_INSTALL_FULL_SYSCONFDIR instead. Changed this in 5020ee6
I agree that it's not a bad idea to look in subdir 'umurmur'. PR is welcome.

[dvzrv]

@fatbob313 I just realized that installing the config to /etc/umurmur/ makes much more sense, as the default configuration also assumes the generated keys in that directory.