Rewrite the INI InventoryParser

The new code parses INI-format inventory files in a single pass using a
well-documented state machine that reports precise errors and eliminates
the duplications and inconsistencies and outright errors in the earlier
three-phase parsing code (e.g. three ways to skip comments). It is also
much easier now to follow what decisions are being taken on the basis of
the parsed data. The comments point out various potential improvements,
particularly in the area of consistent IPv6 handling.

On the ornate marble tombstone of the old code, the following
inscription is one last baffling memento from a bygone age:

-    def _before_comment(self, msg):
-        ''' what's the part of a string before a comment? '''
-        msg = msg.replace("\#","**NOT_A_COMMENT**")
-        msg = msg.split("#")[0]
-        msg = msg.replace("**NOT_A_COMMENT**","#")
-        return msg