Merge Kevin Coffman's keyring ccache branch for Linux, with some modifications:

Merge Kevin Coffman's keyring ccache branch for Linux, with some modifications:

aclocal.m4: Enable keyring ccache if the header and library are available; no
configure-time option.  No error if it's not found.

ccdefname.c: Keep old default of FILE: cache, at least for now.

libkrb5.exports: Don't export krb5_krcc_ops.

ccbase.c: Only initialize krb5int_krcc_mutex if USE_KEYRING_CCACHE; destroy it
in finalization.  Define INITIAL_TYPEHEAD macro (for file vs keyring), and use
it for initialization and in krb5int_cc_finalize.  Re-enable freeing of
additional registered-type structures.

cc_keyring.c: Avoid calls to com_err from within library.

cc_file.c: Punt change; generate_new is badly broken, and we expect to replace
it with a new API anyways.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18638 dc483132-0cff-0310-8789-dd5450dbe970