src/utils_cmd_listval.c: Free memory returned by `uc_get_names'.

src/utils_cmd_listval.c: Free memory returned by `uc_get_names'.

We've run into a memory leak in collectd, triggered by usage of 'listval'
via the unixsock plugin.

When making a 'listval' call, utils_cmd_listval.c:handle_listval() calls
utils_cache.c:uc_get_names() to retrieve a list of active value names from the
internal cache. uc_get_names() uses realloc() to allocate memory in which to
store the list, and returns pointers.

handle_listval() does not perform a free() on the returned memory. Each time
listval is called, some memory is leaked. handle_getval() does not suffer from
the same problem - a free() is called in that case.