slash-cgi SAPI for CGI and FastCGI

As requested in slash-lang/slash#16, I "did the needful" ;) and implemented a SAPI named "sapi-cgi" which, of course, also supports FastCGI.

It probably needs some more testing (I tested it on Linux x86 and x86_64), but it's a start.

It adds a new dependency in case the `--sapi=cgi` is passed to `./configure`: the FastCGI development kit has to be installed.