fix negated return value of ns_skiprr, breakage in related functions

due to a reversed pointer difference computation, ns_skiprr always
returned a negative value, which functions using it would interpret as
an error.

patch by Yu Lu.

add basic dns record parsing functions

based on patch by Timo Teräs, with some corrections to bounds checking
code and other minor changes.

while they are borderline scope creep, the functions added are fairly
small and are roughly the minimum code needed to use the results of
the res_query API without re-implementing error-prone DNS packet
parsing, and they are used in practice by some kerberos related
software and possibly other things. at this time there is no intent to
implement further nameser.h API functions.