pppd: Eliminate some unnecessary ifdefs

pppd: Eliminate some unnecessary ifdefs

Since we only support Linux and Solaris these days, statements like:

#if defined(SOL2) || defined(__linux__)

are always true and can be removed, along with anything in the #else
branch of such a #if, and anything within a #if with the inverse
condition.  Furthermore, inside a #if !defined(__linux__) we know
that SOL2 must be defined.

This simplifies the code along these lines.

Signed-off-by: Paul Mackerras <paulus@samba.org>