sqlfs_instance_count() to get number of active sqlfs instances/threads

Each sqlfs_t_init/sqlfs_open call creates an instance of sqlfs, which is
tracked internally.  This functions allows wrappers to get the number of
active instances to understand the number of active threads.  This is used
to check that an IOCipher VirtualFileSystem can be unmounted.

refs #3402 https://dev.guardianproject.info/issues/3402
zero out buffers that temporarily hold password/key data

refs #3402 https://dev.guardianproject.info/issues/3402
document sqlfs' "init" vs "open/close" modes of operation
base open/close functions on the init functions

In order to use the open/close mode as a way to track the mounted state
of the filesystem, the open/close mode needs to work with the init aka
threaded mode.  That means that the sqlfs instance created by sqlfs_open*
must be assigned to the pthread_key and the filename and password must be
properly cached for any threads that might be created.

refs #3402 https://dev.guardianproject.info/issues/3402