if_sl and if_ppp (from ppp-2.2), eliminating the nearly identical
pppcompress.[ch] code. Add maximum VJ compression states argument to
sl_compress_init().
if_sl: call sl_compress_init() with the extra argument.
- Delete redundant declarations.
- Add -Wredundant-declarations to Makefile.i386 so they don't come back.
- Delete sloppy COMMON-style declarations of uninitialized data in
header files.
- Add a few prototypes.
- Clean up warnings resulting from the above.
NB: ioconf.c will still generate a redundant-declaration warning, which
is unavoidable unless somebody volunteers to make `config' smarter.
