Improve wording for actions when bytelife exceeded

We just want to rekey, not "establish a new context", whatever
that is.  Conveniently, we have a section to link to for this.

Change-Id: I210115c7b6c864729012758a367c8295b7e1c6f8
Add some guidance on nonce lengths for key negotiation

The client nonce is transmitted in the clear and as such does not
really add entropy to the generated master key.  It does help ensure
that the generated key is unique, e.g., if a kerberos service ticket
is used to establish multiple GSS security contexts, though.
Arguably more importantly, the client nonce also is included in the
MIC of the startparams, and as such serves to prevent a replay attack
of the returned MIC in concert with a MITM downgrade attack on the
actual StartParams received by the server.

The server nonce is wrapped in the GSS security context and does
contribute real entropy to the resultant key; however, only as many
bits of entropy as will be used for the key could possibly be useful.

Change-Id: I711d955d9767482de366b2be192dfdcc56c1fce7
Security considerations on nonce lengths

Change-Id: Iacd5db483f3a0f1953e4aa513106a6ff286a25c7