Do not modify c.Opt.MaxSize to pass limits to func
Do not modify c.Opt.MaxSize to pass limits to func

Rather than modifying the c.Opt.MaxSize option to setup the
defaultMaxSize or file Size() limits, instead validate the maxSize and
pass it along to both the non-exported getHash() and compareReader()
funcs.

This will remove the unintuitive behavior of the c.Opt.MaxSize value
being changed by something other than the user, while retaining the
ability of non-regular args to CompareFile() that are limited by the
default or user set MaxSize.  Also user set MaxSize can limit
CompareFile() amount, but if left uninitialized for regular files, the
full file will be compared.

udhos/equalfile,udhos/equalfile