Thread-safety for file-based credentials caches

Thread-safety for file-based credentials caches

* cc_file.c (krb5_fcc_data): Added a mutex.
(krb5_fcc_read*, krb5_fcc_write, krb5_fcc_store_*, krb5_fcc_open_file,
krb5_fcc_skip_header, krb5_fcc_skip_principal): Verify that the mutex is
locked.
(MAYBE_OPEN): Verify that the mutex is locked; unlock it if returning an error.
(krb5_fcc_initialize, krb5_fcc_start_seq_get, krb5_fcc_get_principal,
krb5_fcc_store, krb5_fcc_set_flags): Lock and unlock the mutex.
(krb5_fcc_close): Likewise.  Destroy the mutex when done.
(krb5_fcc_destroy): Merge stdio and non-stdio versions a little more.  Destroy
the mutex when done.
(krb5_fcc_resolve): Initialize and lock the mutex.
(krb5_fcc_next_cred): Lock and unlock the mutex.  Merge the stdio and non-stdio
branches a little more.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16534 dc483132-0cff-0310-8789-dd5450dbe970