config: use the new '_unset' value for 'configlist'
This should let 'configlist' delegate all special processing of the default
config value to the main 'config' method.
The default config value ([]) is still handled in this method.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true