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 UNIX-like file descriptors, and UNIX-like
behavior from the functions: open, close, read, write, lseek.

The quasi-BNF grammar for a credentials cache:

file ::= 
	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

etc.