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