Configurable 'X-Bogosity' message

[Adrian Otto]

I think all of the input in this thread is good. Perhaps the best way to
mitigate the potential problems with stripping some headers out (or not) can
be addressed by adding a new feature: Header Ignore Patterns.

We have talked about ignore-lists recently in the context of evaluating the
spamicity of a given message. At one point we implemented code in lexer.l
that would ignore headers starting with "X-Spam-*". I propose that we take
this one step further. We can use the "pcre" package to allow the site
administrator to supply patterns for words or headers that he/she wants to
ignore by using regex or Perl compatible regular expressions.

I know that creating an external dependency on another package reduces
portability, but in this case, I think pcre is portable enough. By default,
we can include a configuration file with a pattern to ignore X-Spam-*
headers as an example that's commented out. If the administrator want to
filter those headers out, than he/she just comments them out of the config
file, and viola!

Adrian