a) allow blank default prefix
b) select the entire last good prefix is some illegal character used

Thanks to Stefan Knorr (Astron) for hint.

Having it as a subsequent test in wrifterfilter would be less useful;
having it as a unit test in writerfilter is not possible, since
writerfilter does not depend on sw.

From https://github.com/TheJosh/tdf-misc's scrape.php, ported the
concept over to bash, to have it run with the doxygen generation.

.pyc and .pyo binary caches contain the timestamp of the .py file, so for
fedora multilib if the .py files get different times on different arch then the
.py[o|c] files end up being different for the same content

This removes SwFlowFrm::bIsFollow, which is pointless duplication of
m_pPrecede; this fixes lots of assertions (probably some real problem as
well).

The pPrecede member is not maintained properly when setting the
corresponding pFollow member.
The change in SwTxtFrm::JoinFrm() fixes the crash, the other changes
are perhaps fixes for other crashes...
(regression from cc3d0d18)