Bail out if there seem to be no text symbols.

Found by Ævar in an IBM pSeries POWER7 Fedora Linux box.
All the text symbols (code) seemed to be 'D' (data).

There might be something further to fix regarding the use of 'nm'
(or maybe 'ar'?) in this particular box, but this change makes
the test more robust against surprising nm results.
The leading underscore is optional in Darwin.
Use the double-double size only if using long double.

... and only for the buffer size, not for the extraction.
Delete obsolete comment fragment.
AIX/longdouble: switch the default to allow it.

Since the only really unfixable part is the inf/nan support.

Document the troublesomeness in README.aix.
The double-double mantissa precision ... it's complicated.