git-instaweb: improve auto-discovery of httpd and call conventions.

git-instaweb: improve auto-discovery of httpd and call conventions.

This patch allows calling:

   git-instaweb -d apache2

and have the script Do The Right Thing. In particular, the auto-discovery
mechanism has been extended in order to be used for module listing as
well, and the call convention is that if the daemon is apache2/lighttpd
and the parameter to the "-d" option does not end by "-f", the "-f" is
added to the end of the option itself.

Change all backticks to $( ... ) as per Documentation/CodingGuidelines.

Signed-off-by: Flavio Poletti <flavio@polettix.it>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>