Add support to store time offsets in cc_keyring

The code follows the same model used for the memory ccache type.  Time
offsets are stored in each credential cache in a special key just like
the principal name.  Legacy session caches do not store timestamps as
legacy code would fail when iterating over the new offset key.

[ghudson@mit.edu: minor formatting changes; note legacy session
exception in commit message]

ticket: 7768 (new)
target_version: 1.12
tags: pullup
Set expiration time on keys and keyrings

By setting the timeout based on the credetial's timeout we let the
system automatically cleanup expired credentials.

[ghudson@mit.edu: simplified code slightly]

ticket: 7769 (new)
target_version: 1.12
tags: pullup
Remove a warning in AES string-to-key

On 32-bit platforms, the code to translate an iteration count of 0 to
2^32 can trigger a compiler warning.  Since we will basically never
accept an iteration count that high (right now we reject anything
above 2^24), just reject it out of hand.
Remove dangling --with-kdc-kdb-update references

This configure option hasn't done anything since 1.8, so don't mention
it in configure --help or the documentation.  The disable_last_success
and disable_lockout DB options are now used to turn it off.