Fix FHS default sysconfdir
Fix FHS default modulesdir define

The default value of libdir is (unexpanded) '${exec_prefix}/lib'.  In
the non-FHS path this is fine because it only ends up in a variable
where it will be expanded later.  By using this to define modulesdir
we let it slip into a define where it made no sense.
Handle too-short write(3)s in mod_json_cdr

write(3) can write fewer bytes than was requested for any number of
reasons.  The correct behavior is to retry unless there is an error.

If there is an error, try to unlink the file; no sense in leaving
corrupted data laying around.
Output newline after json output in mod_json_cdr

In UNIX, text files by definition end with a newline.