directory handling

[Eric Seppanen]

when calculating the directory we're supposed to use, the following code 
is used:

        if ( strstr(env, "bogofilter") == NULL )
            strcat(dir, BOGODIR);

while this makes sense if we're using $HOME to figure out the directory, 
I don't think it's right if a user is defining $BOGOFILTER_DIR, because if 
a user says 
BOGOFILTER_DIR=/var/bogofilter
the program will use exactly that directory, but
BOGOFILTER_DIR=/var/bf_files
will be second-guessed and we'll actually use /var/bf_files/.bogofilter/
and I don't think we should be second-guessing the user in this fashion.

Unless someone objects I'll look into changing this.