Stop using KRB5_TC_OPENCLOSE

Since KRB5_TC_OPENCLOSE no longer does anything, stop setting it when
we iterate over ccaches.

ticket: 7804
Remove cc_file.c global lookup table

The FILE ccache type maintains a global reference-counted table of
handles, which is perhaps an imperfect workaround for POSIX
per-process file locks.  Remove this table, since we plan to maintain
read fds in cursors and use O_APPEND writes to render locking less
important.
Revamp FILE ccache I/O handling

Remove open file state from the cache handle, use stdio for reading,
use single O_APPEND writes for writing, and use O_CLOEXEC when
opening.  This simplifies the code, fixes some concurrency issues, and
reduces the dependency on POSIX locks (although we still acquire them
for compatibility with older code, or in case O_APPEND writes aren't
concurrency-atomic).

Helper functions change as follows:
* open_cache_file yields a stdio handle, and only opens and locks.
* close_cache_file takes a stdio handle.
* read_header (new) reads the file header and yields a version.
* invalidate_cache and fcc_lseek are no longer needed.
* get_size, read_bytes, and load_bytes operate on a stdio handle.
* read32, read16, load_data, load_principal, and load_cred operate on
  a stdio handle and version.
* write_bytes, store32, store16, and store_principal are no longer
  needed.

fcc_initialize now takes responsibility for writing the header and
principal using a single write.  The cache cursor now stores a stdio
handle to avoid having to reopen the file for each cred, and therefore
now requires a mutex.