* libc/linkaddr.c: Initialized variable to remove warning.
* modem/ppp.c, modem/ppp_tty.c: Made numerous variable declarations
conditional on PPP_COMPRESS and PPP_FILTER. Commented out variables
that were not used because the code using them was commented out.
Removed totally unused variables.
* modem/pppcompress.c: Added parentheses to avoid warnings.
* pppd/pppmain.c: Removed numerous warnings.
