efinet: Defer exclusively opening the network handles

Don't commit to exclusive access to the network device handle by
efinet until the loader has decided to load something through the
network. This allows for the possibility of other users of the
network device.

Submitted by: scottph
Reviewed by: tsoome, emaste
Tested by: tsoome, bcran
Differential Revision: https://reviews.freebsd.org/D20642