Add a sample processor. This talks to spamassassin spamd to find out

Add a sample processor.  This talks to spamassassin spamd to find out
if a message is spam.  If so, it moves it to maildir/spam, else it
moves it to maildir/new.

{libspamc,utils}.[ch] are icky, icky bits from spamassassin's spamc.
The only substantial change is to make libspamc.c:_message_read_raw
mmap the message instead of reading the entire thing into memory (wtf
were they thinking?!).