rp-pppoe 3.8: Fix issues caused by autoconf needing to run in the src

rp-pppoe 3.8: Fix issues caused by autoconf needing to run in the src
subdirectory of the source code. The OE autotools.bbclass runs the autoconf
stuff in ${S}, so you need to change ${S} to the src subdirectory to get it
to work. But ${S} is also where patches are appplied, so you can't patch the
config files and other things which aren't in the src directory. So add some
autoconf at the top level which just says there's a sub-autoconf project in
the src subdirectory, fix up the patches, remove the sed hacks and add a new
patch to do what the sed hacks were doing before.