This change prepends underscores to methods that could have name conflicts with WT internal names. These methods were exposed for conflict because we created public C names by 'extending' classes in SWIG. SWIG's wrapper naming for such methods apparently follows a different convention than the usual "wrap<PythonName><FunctionName>", and uses "<CTypeName><FunctionName", e.g. "__wt_cursor_equals".

The fix is patterned after @agorrod's fix, but macro-ized.

Also includes a dist/s_lang checker to make sure there are no potential conflicts.

Reverts @keithbostic's workaround to rename the conflicting name on the C side.

Refs WT-1574.