buff_t cleanup

[David Relson]

Matthias,

I did a bit of cleaning up of the buff_t code.

Since struct field 'pos' is never used, I removed it.

The t.leng field shows how much data is actually _in_ the buffer and any 
additional data added to the buffer is at the end of the buffer's 
data.  Since this is so, buff_fgetsl() now uses buff->t.leng when it calls 
xfgetsl() to read more text.  This change also allowed removing any code 
(outside of buff.c) that set buff->read.

However, bogofilter does need to know about the most recently read line (to 
save it for passthrough and to print it for debugging), so buff_fgetsl() 
savees the start position of the line in buff->read.

These changes put more of the "bookkeeping code" for buff_t in buff.c and 
simplify the use of the struct.  This should help maintain the code.

David