[krbdev.mit.edu #2147] Debug output hooks need to be added to support Windows Help Desk

[Jeffrey Altman via RT]

I am proposing the following api:

   krb5_error   krb5_get_debug(krb5_context ctx, uint32 *debug_mask)
   krb5_error   krb5_set_debug(krb5_context ctx, uint32 debug_mask)

A mask is used to allow different qualities of debugging messages to be
generated. At first we will probably define a small number of types. 
Perhaps

   KRB5_DEBUG_NONE   0x0
   KRB5_DEBUG_TGS    0x1
   KRB5_DEBUG_AP     0x2
   KRB5_DEBUG_ALL    0xFFFFFFFF

The debugging mode will be stored within the krb5_context.

The initial value of the debugging mode will be read from the profile:
     [libdefaults]
       debug_mask = 3
       debug_log  = FILE:<path>

The actual debug statements themselves will be of the form

    krb5int_debug_message(krb5_context ctx, char * fmt, ...)

Each platform will define its own implementation of
krb5int_debug_message() as needed.   The Windows version will behave
similar to the existing Krb4 interface.  The Unix version could either
write messages to a log file, to stderr, or to the syslog() api.