tool_getparam: Support size modifiers for --max-filesize

- Move the size modifier detection code from limit-rate to its own
  function so that it can also be used with max-filesize.

Size modifiers are the suffixes such as G (gigabyte), M (megabyte) etc.

For example --max-filesize 1G

Ref: https://curl.haxx.se/mail/archive-2017-12/0000.html

Closes https://github.com/curl/curl/pull/2179