This commit updates isc_result_t to be an enum type, thus allowing debugging sessions to promptly identify the string used to define the error number, e.g. ISC_R_NOMEMORY, instead of a number like 37. It does so by having error codes defined in template files located in lib/<libname>/result_<lib>.h.in, those files are combined during the build stage to generate the enum type, stored in lib/isc/include/isc/result_ext.h source file.