mount: use the mount syscall directly

Allow zfs datasets to be mounted on Linux without relying on the
invocation of an external processes.  This is the same behavior
which is implemented for FreeBSD.

Use of the libmount library was originally considered because it 
provides functionality to properly lock and update the /etc/mtab 
file.  However, these days /etc/mtab is typically a symlink to 
/proc/self/mounts so there's nothing to updated.  Therefore, we
call mount(2) directly and avoid any additional dependencies. 

If required the legacy behavior can be enabled by setting the 
ZFS_MOUNT_HELPER environment variable.  This may be needed in
environments where SELinux in enabled and the zfs binary does  
not have mount permission.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Felix Dörre <felix@dogcraft.de>
#10294

man/man8/zfs.8

@@ -710,6 +710,15 @@ Do note that any changes done with the
 .Xr net 8
 command will be undone if the share is ever unshared (such as at a reboot etc).
 .El
+.Sh ENVIRONMENT VARIABLES
+.Bl -tag -width "ZFS_MOUNT_HELPER"
+.It Ev ZFS_MOUNT_HELPER
+Cause
+.Nm zfs mount
+to use
+.Em /bin/mount
+to mount zfs datasets. This option is provided for backwards compatibility with older zfs versions.
+.El
 .Sh INTERFACE STABILITY
 .Sy Committed .
 .Sh SEE ALSO