- mostly doxygen comments
- some alignment

closes #159

Refs #159

Fixed enum dnslib_rr_type - must explicitly set numbers for
  types which do not follow the previous ones in numbering.
Fixed naming of mxrdtln enum items to contain prefix DNSLIB_.

Fixed missing comma.
MAXRDATALEN -> MAX_RDATA_ITEMS
+ added two other constants:
  - MAX_RDATA_ITEM_SIZE
  - MAX_RDATA_WIRE_SIZE

Initial implementation of RR type descriptors. It's more or less a simplified copy from NSD, will need changes and maybe more API functions.