[krbdev.mit.edu #7877] Interleaved init_creds operations use same per-request preauth context

[Greg Hudson via RT]

In preauth2.c, krb5_preauth_context_st mixes context-global preauth data 
with per-request data.  The "tried" field of krb5_preauth_context_st and 
the "req" field of clpreauth_handle are per-request, while the "vt" and 
"data" fields of clpreauth_handle are context-global.

Alll of this information is managed out of krb5_context, not 
krb5_init_creds_context.  So if a caller interleaves 
krb5_init_creds_init/krb5_init_creds_step for different 
krb5_init_creds_context objects with the same krb5_context object, both 
operations will use the same per-request preauth data, likely causing one 
or both operations to fail.

The per-request information should be separated out and managed out of 
krb5_init_creds_context.