[krbdev.mit.edu #2275] add encrypt_size_limit to crypto API

[Ken Raeburn via RT]

(See also ticket 2266 for background discussion.)

The GSSAPI gives applications a function wrap_size_limit to determine
how large a message can be encoded into a given output buffer size.  Our
crypto API provides no such function, so the GSSAPI code has to loop
calling encrypt_size or encrypt_length until it finds the boundary
between small-enough and too-big.

If we add an encrypt_size_limit type function to the crypto API, the
GSSAPI code would be much more efficient.