This commit automatically generates two files during compile time,
which will be used by the new result API implementation.
lib/isc/result_ext.c: result of combining lib/*/result_*.c.in files.
lib/isc/include/isc/result_ext.h: result of combining
lib/*/result_*.h.in files.
result_ext.c will be included by lib/isc/result.c to have all relevant
symbols available during compile time, this substitutes the old method
of having each library calling <libname>_result_register() functions.
result_ext.h will be included by lib/isc/include/isc/result.h, so source
files can now include only <isc/result.h> to have all the ISC result
codes available.