Zero, <TYPE>_MIN and <TYPE>_MAX values can result from valid conversions.
They don't necessarily signal any error.
Since we do not have any reliable error signalling from libkern's strto*(),
it's better to always assume success rather than to report an error when
there is none.
@tsoome suggested that for strtol and strtoul we can internally use
strtoq and strtouq, so that we can check a result for being within
the corresponding ranges. Not sure if it's worth the trouble.
There is no equivalent technique for strtoll and strtoull anyway.