Refactor the isc_buffer_{get,put}uintN, add isc_buffer_peekuintN

Refactor the isc_buffer_{get,put}uintN, add isc_buffer_peekuintN

The Stream DNS implementation needs a peek methods that read the value
from the buffer, but it doesn't advance the current position.  Add
isc_buffer_peekuintX methods, refactor the isc_buffer_{get,put}uintN
methods to modern integer types, and move the isc_buffer_getuintN to the
header as static inline functions.