configuration revamp

- make many more things configurable
- stop using ::params::* for things that are configurable
- add worker_rlimit_nofile option
- add tcp_nopush option
use return instead of rewrite for simple redirects (e.g., www -> naked domain)
Merge pull request #314 from janorn/pull_269

Pull request 269 revisited
Merge pull request #349 from rabbitt/feature/return-instead-of-rewrite

use 'return' over 'rewrite'