In _reader we were raising OSError(errno=ENOSYS), but in _dameon
we were raising NotImplementedError. Let's always use ENOSYS.

Also, make the messages in _reader more specific.

Fixes #33.

Let's pass any flags through to the journal functions without checking
validity. Those functions do their own checking, so there's no need to
second-guess.

The semantics for _Reader(flags=0) are a bit changed:
before, this would be transformed into sd_journal_open(SD_J_LOCAL_ONLY).
Now, this results in sd_journal_open(0). Previous behaviour should be
achieved by not specifying the flags at all.
This change is necessary, because previously it was not possible to
pass flags=0 to sd_journal_open(0), i.e. it was not possible to "merge"
journals through the Python interface.

Similarly, Reader(flags=0) now means to open all journals, and
Reader(flags=None) is the same as Reader(flags=LOCAL_ONLY).

Those are field names and they should always be ASCII, and converting
them to str automatically makes the answer more useful.