units: bring std::abs in line with bounds_checked_int abilities
units: bring std::abs in line with bounds_checked_int abilities

(needed for some code.  IIRC we avoid unqualified abs() because
it misses std overloads for non-'double' types.. possibly even
calling the 'double' version [which is specified by C] for ints)

Still a bit dubious, ah well.

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