usr.sbin/autofs: Best effort to maintain mounttab and mountdtab

from FreeBSD 88e531f38c2412bf030f4e8dd563efc45b70797e

usr.sbin/autofs: Cleanup workaround code for a VFS bug

autofs: Minimize non functional diff vs FreeBSD

sys/sys/mount.h: Add fsidcmp()

Only used by autofs at the moment.

from freebsd/freebsd@30596480c966b8d8bc7436df3467ab2eac145751

usr.sbin/autofs: Change autounmountd(8) to use time_t for duration instead of double

freebsd/freebsd@d1115235daf2a6a2f3bcdb3c840722fbc548d2ee

autofs: main_auto{,un}mountd() never return, so do not pretend they do.

usr.sbin/autofs: Make autounmountd(8) not die when traced with "truss -p". (FreeBSD@GitHub 8381908c)

usr.sbin/autofs: Use proper argument order for calloc(3). (FreeBSD@GitHub 6add75fe)

usr.sbin/autofs: Workaround namecache bug after unmount

autounmountd gets affected by the namecache bug mentioned in
https://bugs.dragonflybsd.org/issues/2908.

This can be worked around by stat(2)

usr.sbin/autofs: Workaround namecache bug after unmount

autounmountd gets affected by the namecache bug mentioned in
https://bugs.dragonflybsd.org/issues/2908.

This can be worked around by stat(2) (or any syscall that resolves
the name once again) after unmount failed with EBUSY.

Without this workaround, a process at automounted filesystem will
see ENOTCONN after autounmountd's attempt to unmount. This must
not be removed until the namecache bug is fixed.

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autofs: Port autofs from FreeBSD

Brought in basically from
FreeBSD@GitHub cac9beab7d53f0c37ce2a2a1b893be59028928f4
with lots of changes.
Note that this commit isn't necessarily 1:1 with above commit

autofs: Port autofs from FreeBSD

Brought in basically from
FreeBSD@GitHub cac9beab7d53f0c37ce2a2a1b893be59028928f4
with lots of changes.
Note that this commit isn't necessarily 1:1 with above commit.

Kernel code is basically a rewrite based on the FreeBSD code.
Userspace is basically 1:1 with FreeBSD except that lots of small
changes (including related commits listed below) were necessary.
This is due to autofs being dependent on FreeBSD specific interface,
command options and such.

For userspace, note that non-functional stuff (e.g. whitespace
warnings via git am) are intentionally left to be 1:1 with FreeBSD.
Userspace is basically portable, so don't try to obfuscate the real
changes made for DragonFly by fixing these for now till things are
considered stable unless it's a bug from FreeBSD.

Summary of newly added or modified files.
- sys/vfs/autofs - autofs filesystem
- usr.sbin/autofs - autofs userspace command and daemons
- etc/ - configuration files and manpages
- others - changes in misc subsystems (not independent of autofs)

Related DragonFly commits.
- usr.sbin/autofs: Workaround namecache bug after unmount
- sys/kern: Don't implement .vfs_sync unless sync is supported
- user.sbin/fstyp: Port fstyp from FreeBSD
- sys/kern: Retry nlookup if nresolve returned ESTALE
- sys/sys: Fix IOCPARM_MAX
- sys/sys: Extend IOCPARM_MAX
- usr.bin/showmount: Add -E option
- sbin/mount_nfs: Add -o retrycnt= option
- sys/kern: Add kqueue EVFILT_FS
- sys/kern: Add kstrndup()

Related DragonFly PRs.
- https://bugs.dragonflybsd.org/issues/2900
- https://bugs.dragonflybsd.org/issues/2901
- https://bugs.dragonflybsd.org/issues/2905
- https://bugs.dragonflybsd.org/issues/2907
- https://bugs.dragonflybsd.org/issues/2908
- https://bugs.dragonflybsd.org/issues/2909
- https://bugs.dragonflybsd.org/issues/2912
- https://bugs.dragonflybsd.org/issues/2913
- https://bugs.dragonflybsd.org/issues/2914

Other related resource.
- http://lists.dragonflybsd.org/pipermail/users/2016-May/thread.html#249556
- http://lists.dragonflybsd.org/pipermail/users/2016-June/thread.html#249680
- https://www.dragonflydigest.com/2016/05/06/18066.html

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