ipfw: fix jail option after r348215

r348215 changed jail_getid(3) to validate passed-in jids as active jails
(as the function is documented to return -1 if the jail does not exist).
This broke the jail option (in some cases?) as the jail historically hasn't
needed to exist at the time of rule parsing; jids will get stored and later
applied.

Fix this caller to attempt to parse *av as a number first and just use it
as-is to match historical behavior. jail_getid(3) must still be used in
order for name arguments to work, but it's strictly a fallback in case we
weren't given a number.

Reported and tested by: Ari Suutari <ari stonepile fi>
Reviewed by: ae
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21128