Implemented limit-rate feature

Basic implementation of limit-rate feature (as described in docs/wget2.md)

Notes:

• Set by command-line argument --limit-rate
• The limit rate is split and shared equally among the download threads

Issues:

• There is no adjustment if one of the download threads is downloading slower than the limit-rate.
• The sharing of the limit-rate includes download threads without active jobs.

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code