parser: add 'delete map' syntax

parser: add 'delete map' syntax

Creating a map is done via "add map".  However, to delete a map requires using
"delete set", which is confusing.  Add the appropriate synonym to parser.
The downside to this is that one can now delete a set with "delete map", but
this seems a minor issue.  It could of course be fixed by adding a new
CMD_OBJ_MAP.

This closes netfilter bugzilla #879.

Reported-by: Bjørnar Ness <bjornar.ness@gmail.com>
Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>