Cleanup in pppd/pppd.h, eliminate unecessary headers

This removes the need to include the following heades in pppd.h

<limits.h>, this is included where needed (main.c). The number of groups already retrieved is stored in the "int ngroups" variable.
<sys/params.h>, use MAXPATHLEN where needed
<net/if.h>, such that the value of IFNAMSIZ doesn't have to be declared to include <pppd/pppd.h>

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