Replace G_DIR_SEPARATOR by / this is easier to read.
Allow to set empty download-dir.

If the download directory is not set or empty the download will be put into
current directory.
Don't expand download-path if set.

It is unusual to expand the variables when they are set. This will irritate
the user in case ':set download-path=$HOME' is set but ':set download-path?'
spits out not what the user set before.
So vimb follows the example given by vim and save the path like is was given
and expands it it it's used.
Added missed precompiler defs for cookie feature.
Removed default.h and set default settings in init_settings().

This saves some time spent on parsing the string values into the setting args.