Read atof(lineptr) into a variable for future repetitive use.

Soon, support of --max will require reading atof(lineptr) multiple times.
Better put it into a variable.
Calculate maximum number.

Not enabled yet from the command line. See next commit for that.
Add test for --max/-M and make it pass by adding support for it.
Add support for --min/-m with test and everything.

Use the same changes needed for --max.