Wrap readline oursleves to avoid py3's interference

Wrap readline oursleves to avoid py3's interference

We don't want Python to "help" us by guessing and insisting on the
encoding before we can even look at the incoming data, so wrap
readline ourselves, with a bytes-oriented (and more direct) API.  This
will allows us to preserve the status quo for now (and maintain parity
between Python 2 and 3) when using Python 3 as we remove our LC_CTYPE
override.

At least on Linux, readline --cflags currently defines _DEFAULT_SOURCE
and defines _XOPEN_SOURCE to 600, and the latter conflicts with a
setting of 700 via Python.h in some installations, so for now, just
defer to Python as long as it doesn't choose an older version.

Thanks to Johannes Berg for fixes for allocation issues, etc. in an
earler version, and help figuring out the #define arrangement.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Rob Browning <rlb@defaultvalue.org>