With locking disabled (if gdbm_open was called with GDBM_NOLOCK),
the user may want to perform their own file locking on the database file
in order to prevent multiple writers operating on the same file
simultaneously.
In order to support this, the gdbm_fdesc routine is provided.
Returns the file descriptor of the database dbf. This value
can be used as an argument to flock, lockf or similar
calls.