gitweb: Run in FastCGI mode if gitweb script has .fcgi extension

If the name of the script ($SCRIPT_NAME or $SCRIPT_FILENAME CGI
environment variable, or __FILE__ literal) ends with '.fcgi'
extension, run gitweb in FastCGI mode, as if it was run with
'--fastcgi' / '--fcgi' option.

This is intended for easy deploying gitweb using FastCGI
interface.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>