Add some input validation.

Add some input validation.

Our raw data doesn't have the qualitiy one should
expect. There are empty header lines only containing
whitespace (leading to wrong joining of apparent
continuation lines); header lines that contain garbage
without ':' so split is failing; empty 'newsgroups'
fields; unsupported encondings in MIME encoded words
... and so on.

Add fixes for the aforementioned problems.

Signed-off-by: Thomas Hochstein <thh@inter.net>