Port autofs to DragonFly

[Tomohiro Kusumi]

I've been working on porting autofs from FreeBSD for the last 3-4 weeks.
It finally started to work in the last few days though it's still *very* buggy.

I'll be committing this to master sometime in May even if it's not
100% stable after tested by a user who asked me to port this, as I
don't want to spend too much time on this.
I don't use NFS, but it's easy to imagine issues with NFS just like
other filesystems.

It actually took more time than I thought it would be, because there
were several features supported by FreeBSD but not on DragonFly,
which broke portability of userspace and made me bring in those
missing stuff from FreeBSD first.

(I did this as a practice before I attempt to port HAMMER1 to other BSD kernel.
I originally planned to do this with ext2, but changed my mind as
someone asked me to port autofs.)

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[root@]~# uname -r
4.5-DEVELOPMENT
[root@]~# kldload autofs
[root@]~# grep "/mnt" /etc/auto_master
/mnt            -media          -nosuid
[root@]~# ls /mnt
[root@]~# automount
[root@]~# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
map -media         0    0     0   100%    /mnt
[root@]~# automountd -vvvvvvvv
[root@]~# ls /mnt
Fedora-S-23-x86_64
[root@]~# cd /mnt/Fedora-S-23-x86_64
[root@]/mnt/Fedora-S-23-x86_64# ls
.discinfo .treeinfo EFI       Packages  TRANS.TBL images    isolinux  repodata