Fix readline/libedit selection code to prefer readline over libedit

reliably (ie, regardless of which libraries they depend on).  Also
make sure that we don't select headers that obviously belong to the
wrong one of the two libraries.  This was discussed back around 4-Sep
but seems to have slipped through the cracks.  The header selection
could be checked more closely, perhaps, but let's see if this is good
enough.