Change env only if readline could not find inputrc

If the user has an inputrc file that readline could find (e.g. ~/.inputrc)
then just use that.  For users who don't have that, set things up like we have
in the past so that readline can find clink_inputrc_base and clink_inputrc.
Also fixed hooked_stat to set errno on failure, so that we can tell when
reading inputrc failed.

resolve mridgers/clink#257
resolve mridgers/clink#258
Add default inputrc, and generate clink_inputrc_base at build time

Users can include the default inputrc from their own inputrc file.  The
clink_inputrc_base file is now generated by copying the default inputrc file
and appending '$include ~/clink_inputrc' to the end.