ctld: Defer initialization of NVMeoF associations

Wait until all of the configuration has been parsed before creating
associations for NVMe portals. This ensures that any options
specified in a transport group are honored when creating associations.

To enable this, add a new virtual method portal::prepare invoked when
applying a configuration prior to opening a socket (or reusing an
existing socket) for a portal.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D52844