add support for CGI

[kazuho]

H2O now supports CGI, with the addition of share/h2o/fastcgi-cgi, a daemon that runs as a FastCGI server and spawns CGI scripts.

For example, with this PR it is possible to execute CGI scripts by defining a custom handler for .cgi extension:

file.custom-handler:
  extension: .cgi
  fastcgi.spawn:
    command: "exec share/h2o/fastcgi-cgi"

Still WIP, need to add max proc. restriction, etc.

[kazuho]

ToDo:

• export H2O_ROOT from core, so that the spawn command can be defined as: exec $H2O_ROOT/share/h2o/fastcgi-cgi
• implement max proc. restriction
• add tests
  • add minimal tests
  • add test using STDIN
  • add test using STDERR