Changed DFM from open-at-first-access to explicit-open. Before,

Changed DFM from open-at-first-access to explicit-open.  Before,
calling dfm_get_record() or dfm_put_record() would automatically
open the file.  Now, you have to call dfm_open_for_reading() or
dfm_open_for_writing() explicitly.  This makes it possible to
check permissions, file existence, etc. earlier.

Also made struct file_handle more opaque, and clean up in general.

Fixed cmd_parse() so that it always skips past a full command
name.  A few special commands for which this would be bad get
special treatment.  This lets us drop code for skipping past the
end of a command name in most cmd_*() functions.  It's not worth
listing all the commands affected.

Start work on better test framework.

Fix memory leaks.

Misc other changes.