Cleanup the dns_name macros

Cleanup the dns_name macros

1. Replace the "high-performance" macros that were only used if
   DNS_NAME_USEINLINE was defined before including <dns/name.h> with
   inline header functions with assertion checks and thus use them
   everywhere.

2. Replace the old struct initializers with C99 designated initializers
   for better understanding what is happening in these macros.