SAE: Centralize function for sending initial COMMIT

SAE: Centralize function for sending initial COMMIT

When performing SAE authentication in mesh, one station may
initiate authentication by sending a COMMIT as soon as a peer
candidate is discovered. Previously we did this in mesh_rsn.c,
but this left some of the state initialization in a different
part of the code from the rest of the state machine, and we may
need to add other initializations here in the future, so move
that to a more central function.

Signed-off-by: Bob Copeland <me@bobcopeland.com>