Pull the "linux-include" versions of "slip.h" and "slcompress.h" up to

Pull the "linux-include" versions of "slip.h" and "slcompress.h" up to
the top-level directory, remove from them all stuff not needed by
tcpdump, include them rather than the system's header files, and get rid
of assorted unnecessary include files.

This means you don't need to check for "slip.h" when building
"print-sl.c", stubbing it out on systems that don't have "slip.h".  We
also don't need to build the stuff in "print-vjc.c" only on BSD/OS.

Use "u_int32_t" and "u_int16_t" rather than "u_int" and "u_short" for
structure members in "print-chdlc.c".

Move PPP_HDRLEN from "print-ppp.c" to "ppp.h", as per the XXX comment in
"print-ppp.c".