Trace logging

Trace logging

Add trace logging infrastructure code, enabled by the KRB5_TRACE
environment variable or the API functions krb5_set_trace_callback()
or krb5_set_trace_filename().  As a start, add tracing events for:
  * AS-REQ client code, including FAST and preauth
  * TGS-REQ client code
  * AP-REQ and AP-REP code (client and server)
  * sendto_kdc
  * Selected ccache operations
  * Selected keytab operations

ticket: 6737

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24118 dc483132-0cff-0310-8789-dd5450dbe970