pofile parsing. Fix handling of obsolete + refactor

pofile parsing. Fix handling of obsolete + refactor

Fixes the handling of the unit before the obsolete unit.
Previously it would mark the unit before an obsolete unit as obsolete
also.

Some refactoring:
* the transition between finishing one unit and starting the next
is clearer
* separate the processing of keywords and continuation lines
* combine the reset and initialisation code
* Make the handling of strings consistent.
* Add some nascent error handling, removed some errors in test inputs