Linux's struct stat does not have an st_flags member, but we still want
to be able to create FreeBSD disk images with non-zero st_flags. Add a
replacement member to struct fsinode to store it on Linux.

Note that we add a fallback definition of HAVE_STRUCT_STAT_ST_FLAGS for
makefs.h since etdump reaches into makefs and won't have it defined in
its CFLAGS like makefs itself does, and on FreeBSD we don't provide an
nbtool_config.h.

Note also that strtofflags is currently a stub on Linux so this doesn't
yet properly mirror the mtree, but that will change in a future commit.