gitweb: support automatic FCGI detection
Add support for a new configuration option '$auto_fcgi' and when
true automatically activate FCGI mode when getsockname succeeds on
STDIN.
Enabling this feature allows the same configuration to be used for
both non-FCGI (i.e. regular CGI mode) and FCGI mode operation.
This feature is always opt-in only. Because although unlikely, it's
certainly feasible for some hypothetical server software to run
normal CGI mode programs with STDIN connected to a socket for which
getsockname succeeds.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>