* cc_file.c: Add buffering on reading.

* cc_file.c: Add buffering on reading.
(FCC_BUFSIZ): New macro.
(struct _krb5_fcc_data): Add new fields buf, valid_bytes, cur_offset.
(krb5_fcc_resolve, krb5_fcc_generate_new): Initialize valid_bytes.
(invalidate_cache): New function.
(krb5_fcc_write, krb5_fcc_open_file, krb5_fcc_destroy): Call invalidate_cache.
(fcc_lseek): New function.
(krb5_fcc_skip_header, krb5_fcc_destroy, krb5_fcc_start_seq_get,
krb5_fcc_next_cred, krb5_fcc_store): Use fcc_lseek instead of lseek.
(fcc_read): Use and maybe refill the buffer.
(dereference): Zap the contents of the buffer before freeing it.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16666 dc483132-0cff-0310-8789-dd5450dbe970