The function `pidfds_to_pids` expected `-1` if `open_fdinfo_dir` failed but the latter returned `errno` which is hard to distinguish from a valid file handle. Correct that by making `open_fdinfo_dir` a wrapper around `open`.
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.
