mem_maybedup() can no longer fail

mem_maybedup() can no longer fail

mem_maybedup() calls isc_mem_allocate() if an mctx is supplied,
but that can no longer fail, so now the only way mem_maybedup()
could return NULL is if it was given a NULL source address by the
caller. this commit adds a REQUIRE to prevent that scenario, and
cleans up all the calling code that previously checked for NULL
return values.

this function is mostly used in rdata tostruct() implementations, so
the documentation for dns_rdata_tostruct() has been updated to
remove 'ISC_R_NOMEMORY' as a possible return value.