Add functions to open pidfds, i.e. file descriptors representing
processes, for process ids and vice versa. Both functions work
an array of fds/pids, stop on error and return the number of
successfully handled items.
Much like the auto-closing helper for file descriptors, add a new
auto-free helper that is meant to be used with dynamically allocated
memory, a la:
autofree char *data = NULL;
...
data = malloc(size);
When data goes out of scope, cleanup_free will be called with &data,
i.e. cleanup_free(&data), which in turn will call free(3) data. In
order to work with all types, e.g. 'char *' (resulting in char **
being passed to cleanup_free) or 'int *' (resulting in int ** being
passed to cleanup_free), cleanup_free is defined to work with void *,
hence the odd-looking cast: void *target = *(void **) ptr.
Specify the include directory in the link_daemon_common dependency
and thus everything that includes that as a dependency will get
the proper include directory automatically.
