Make the Base object accessible from the toplevel 'dnf' module.
cosmetic: trailing whitespace in dnf.yum.parser
YumOptionParser._non_nones2dict()
BaseConfig.overrides()
Get rid of preconf.

Also simplify config loading into YumBase.read_conf_file(), leaving most of the
mess out of there and in dnf.yum.config.

Also YumBase is now already initialized with a ready-to-use conf object
(containing the defaults), no lazy trigger is involved for base.conf.
remove base.yumvar property.

Make yumvar only accessible through base.conf, it is closely entangled and
created in pretty much in the same place.
rename 'YumBase' to 'Base'.

However this program is called in the future, a Base is a Base.
cosmetic: trailing whitespace in dnf.yum.callbacks