cut: better input validation for -b and -f

[mknos]

• Options -b and -f are consistent in how the value is handled
• Byte and field numbers count from 1
• Negative numbers are treated as a range end, with range starting from 1
• Regular ranges have 2 numbers separated by '-'
• Introduce a number validation function
• test1: positive -b value: "echo 'a:b:c:d' | perl cut -b 3" --> byte 3 is "b"
• test2: negative -b value: "echo 'a:b:c:d' | perl cut -b -3" --> bytes 1-3 are "a:b"
• test3: -b range: "echo 'a:b:c:d' | perl cut -b 2-3" --> bytes 2-3 are ":b"
• test4: positive -f value: "echo 'a:b:c:d' | perl cut -d ':' -f 2" --> field 2 is "b"
• test5: negative -f value: "echo 'a:b:c:d' | perl cut -d ':' -f -2" --> fields 1-2 are "a:b"
• test6: -f range: "echo 'a:b:c:d' | perl cut -d ':' -f 2-3" --> fields 2-3 are "b:c"

[briandfoy]

I think there are a few formatting things to fix up, but I'll take care of that. Maybe it's just the GitHub display of the diff.