Adding an ability to limit max number of concurrent forks

* to the best of my ability the lib is backwards compatible
* main.go would be not able to compile with old library
* all fork denials end up in HTTP 429 response
* new commandline option called --maxforks and affects CGI and ws handlers
Merge between maxforks and passenv feature implementations
Merge pull request #54 from asergeyev/maxconn

Limiting max number of forks #46