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

docs/cmdline-opts/limit-rate.d

@@ -8,7 +8,7 @@ your transfer not to use your entire bandwidth. To make it slower than it
 otherwise would be.
 
 The given speed is measured in bytes/second, unless a suffix is appended.
-Appending 'k' or 'K' will count the number as kilobytes, 'm' or M' makes it
+Appending 'k' or 'K' will count the number as kilobytes, 'm' or 'M' makes it
 megabytes, while 'g' or 'G' makes it gigabytes. Examples: 200K, 3m and 1G.
 
 If you also use the --speed-limit option, that option will take precedence and

docs/cmdline-opts/max-filesize.d

@@ -7,6 +7,10 @@ Specify the maximum size (in bytes) of a file to download. If the file
 requested is larger than this value, the transfer will not start and curl will
 return with exit code 63.
 
+A size modifier may be used. For example, Appending 'k' or 'K' will count the
+number as kilobytes, 'm' or 'M' makes it megabytes, while 'g' or 'G' makes it
+gigabytes. Examples: 200K, 3m and 1G. (Added in 7.58.0)
+
 \fBNOTE:\fP The file size is not always known prior to download, and for such
 files this option has no effect even if the file transfer ends up being larger
 than this given limit. This concerns both FTP and HTTP transfers.