implement --max-keepalive-requests (issue #124)

No tests for this change. One problem is that test_psgi() does not offer an easy way to set the server options (or I did not found out how). Here's a seperate commit which implements a test with too much code for my taste: https://github.com/eserte/Starman/commit/0dbde80b8c3f889ef71b3cc07c30842db6945223

The change may be manually tested using strace:

start the server with your favorite app and some --max-keepalive-requests and --max-requests setting:
```
perl -Mblib script/starman --max-keepalive-requests=7 --max-requests=3 your_favorite.psgi
```
run a keep-alive client with strace just filtering the connect() syscalls --- depending on the server configuration there should be just one connect() to the server (no --max-keepalive-requests setting, --max-requests setting irrelevant) or many connect() calls
```
strace -econnect perl -MLWP::UserAgent -e '$ua=LWP::UserAgent->new(keep_alive=>1); for (1..9) { die if !$ua->get("http://localhost:5000")->is_success }'
```

miyagawa / Starman