If OPENCLOSE is defined, ecah of the functions opens and closes the
file whenever it needs to access it.  Otherwise, the file is opened
once in initialize and closed once is close.

This library depends on ANSI C library routines for file handling.  It
may also have some implicit assumptions about UNIX, but we'll get
those out as much as possible.

If you are running a UNIX system, you probably want to use the
UNIX-based "file" cache package instead of this.

The quasi-BNF grammar for a credentials cache:

file ::= 
	format-vno principal list-of-credentials

credential ::=
	client (principal)
	server (principal)
	keyblock (keyblock)
	times (ticket_times)
	is_skey (boolean)
	ticket_flags (flags)
	ticket (data)
	second_ticket (data)

principal ::= 
	number of components (int32)
	component 1 (data)
	component 2 (data)
	...
	
data ::=
	length (int32)
	string of length bytes

format-vno ::= <int16>

etc.