Add prependUint{16,32} to asn1.Buffer

To mirror DataView. Also rename prepend to mirror DataView.
Unfinished GSS-API stuff
Have constants for TOK_ID values
Pick a key usage

That's probably the right one? I guess we'll find out. I can't find
anything in the specs.
More makeAPReq API changes
Support generating a random sequence number
Add a randomToKey function in the DES enctypes
And implement a krb.Key.makeRandomKey
And implement a useSeqNumber attribute
Better network error handling
Fix krb.Key.makeRandomKey
Generate an initial context token

xvm-remote.mit.edu's remctl server seems to accept it. Still needs all
kinds of machinery around actually be a reasonable GSS-API
implementation. But here's the bare minimum around of glue that's
vaguely functional.