parser: extend limit syntax

parser: extend limit syntax

The documentation describes the syntax of limit statements thus:

  limit rate [over] packet_number / TIME_UNIT [burst packet_number packets]
  limit rate [over] byte_number BYTE_UNIT / TIME_UNIT [burst byte_number BYTE_UNIT]

  TIME_UNIT := second | minute | hour | day
  BYTE_UNIT := bytes | kbytes | mbytes

From this one might infer that a limit may be specified by any of the
following:

  limit rate 1048576/second
  limit rate 1048576 mbytes/second

  limit rate 1048576 / second
  limit rate 1048576 mbytes / second

However, the last does not currently parse:

  $ sudo /usr/sbin/nft add filter input limit rate 1048576 mbytes / second
  Error: wrong rate format
  add filter input limit rate 1048576 mbytes / second
                   ^^^^^^^^^^^^^^^^^^^^^^^^^

Extend the `limit_rate_bytes` parser rule to support it, and add some
new Python test-cases.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>