src: use start condition with new destroy command

src: use start condition with new destroy command

tests/py reports the following problem:

any/ct.t: ERROR: line 116: add rule ip test-ip4 output ct event set new | related | destroy | label: This rule should not have failed.
any/ct.t: ERROR: line 117: add rule ip test-ip4 output ct event set new,related,destroy,label: This rule should not have failed.
any/ct.t: ERROR: line 118: add rule ip test-ip4 output ct event set new,destroy: This rule should not have failed.

Use start condition and update parser to handle 'destroy' keyword.

Fixes: e1dfd5cc4c46 ("src: add support to command "destroy")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>