Parse INI config

Parse a tiny INI-like config given as a single string. Lines are separated by '\n'. A line that is blank or starts (after no leading spaces) with '#' is ignored. A line of the form '[name]' opens a section. A line of the form 'key=value' assigns within the current section. Leading/trailing spaces around key and value are stripped; the value may itself contain '=' (split only on the first). Keys before any section header go into a section named ''. Later assignments to the same key in the same section overwrite earlier ones. Return a dict mapping section name to a dict of its key/value strings.