Add $(INSTDIR) to installation paths in Linux makefiles.

Patch from Robert Vogelgesang.

This patch re-introduces the possibility for package maintainers to
install into a different filesystem tree. This is done by adding
$(INSTROOT) at various places; the package maintainers can then
issue a

        make install INSTROOT=/some/where/else

when they package pppd for their distribution.  In previous versions
of the ppp package this was possible using the variable DESTDIR, but
this variable is now used for a different purpose and cannot be
changed via additional parameters of the make command.