lib/fileutils: add close_all_fds()

lib/fileutils: add close_all_fds()

The classic way which is based on file-descriptors table size is
pretty expensive (due to table size) and forces code to do many
unnecessary close() calls. It seems better to use /proc/self/fds and
close used descriptors only.

Addresses: https://github.com/karelzak/util-linux/issues/883
Signed-off-by: Karel Zak <kzak@redhat.com>