gitweb: Add support for FastCGI, using CGI::Fast

gitweb: Add support for FastCGI, using CGI::Fast

Former run() subroutine got renamed to run_request().  The new run()
subroutine can run multiple requests at once if run as FastCGI script.

To run gitweb as FastCGI script you must specify '--fastcgi' / '-f'
command line option to gitweb, otherwise it runs as an ordinary CGI
script.

[jn: cherry picked from 56d7d436644ab296155a697552ea1345f2701620
 in http://utsl.gen.nz/gitweb/?p=gitweb which was originally based
 on v264 (2326acfa95ac86a53804ca8eeeb482c2f9265e34) by Kay Sievers;
 updated to reflect current gitweb code]

TODO: update 'gitweb/README' and/or 'gitweb/INSTALL' files.

Signed-off-by: Sam Vilain <sam.vilain@catalyst.net.nz>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>