For Linux, use the Linux / Glibc based defines instead of included headers

For Linux, use the Linux / Glibc based defines instead of included headers

This is to ensure compatibility with the OS you are compiling against and that
headers are maintained in upstream projects.

- Moved PPP_EAP and PPP_ECP into respective header files in lieu of not currently
  existing in the linux/ppp_defs.h

- Unchained the top-level ${topsrc_dir}/include, this folder is included for
  prosterity and may continue to exist on github, but in the future eliminated from
  distribution

- Bogus upstream file in glibc for <net/if_ppp.h>, its content should be replaced
  with a simple include to <linux/ppp-ioctl.h>. The lack of an appropriate ifreq
  structure with ppp_stats or ppp_comp_stats, implementet that inline (and tested).

- Updated instances where PPP_FCS() macro would expand the fcstab, while PPP_GOODFCS
  and PPP_INITFCS is provided in <linux/ppp_defs.h>, the latter is tied to a lookup
  table. It's used in two places, so add the PPP_FCS macro where applicable.

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>