timeadd: make static

ares__timeadd() was only ever used from within the same source
timeoffset: made static and private

ares__timeoffset() was only used once within this single source file
nowarn: use <limits.h> instead of configure for size of long

This makes the header file much more multi-arch friendly and can be used
as-is with both 32 bit and 64 bit builds.
ares_rules.h: CARES_SIZEOF_LONG doesn't exist anymore, don't test for it

It was removed in f19387dd72432
Fixed warning 'type specifier missing'.