gitweb: fallback to system-wide config file if default config does not exist

gitweb: fallback to system-wide config file if default config does not exist

From a distribution point of view, configuration files for applications
should reside in /etc/.  On the other hand it's convenient for multiple
instances of gitweb (e.g. virtual web servers on a single machine) to have
a per-instance configuration file, just as gitweb currently supports
through the file gitweb_config.perl next to the cgi.

To support both at runtime, this commit introduces GITWEB_CONFIG_SYSTEM as
a system-wide configuration file which will be used as a fallback if the
config file sprecified throug GITWEB_CONFIG does not exist.

See also
 http://bugs.debian.org/450592

Signed-off-by: Gerrit Pape <pape@smarden.org>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>