git-instaweb: Extract configuring web server into configure_httpd

This is preparatory work for making start/restart check that
git-instaweb set up correct configuration, and generate it if it is
missing.

Pure refactoring, no functional changes.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>