tool: support fractions for --limit-rate and --max-filesize

Allow 2.5k or 3.7M etc. Add mention in documentation.

Verify in test case 1623.

Closes #20266

src/tool_getparam.h

@@ -375,6 +375,7 @@ ParameterError getparameter(const char *flag, const char *nextarg,
 ParameterError parse_cert_parameter(const char *cert_parameter,
                                     char **certname,
                                     char **passphrase);
+UNITTEST ParameterError GetSizeParameter(const char *arg, curl_off_t *out);
 #endif
 
 ParameterError parse_args(int argc, argv_item_t argv[]);