git-instaweb: Check that correct config file exists for (re)start

git-instaweb: Check that correct config file exists for (re)start

Currently start/restart does not generate any configuration files for
spawning a new instance.  This means that

  $ git instaweb --http=<server> --start

might pick up stale 'httpd.conf' file for a different web server
(e.g. for default lighttpd when requesting apache2).

This commit changes that, and makes git-instaweb generate web server
config file and/or gitweb config file if don't exists.

This required naming config files after the name of web server
(alternate solution would be to somehow mark for which web server was
config file generated).

Note that web servers that embed configuration in server script file,
namely webrick and plackup, and which delete "$conf" in their *_conf
function, would have their config (server script) always regenerated.

Note: this commit introduces a bit of code repetition (but only a few
lines).

Reported-by: Gurjeet Singh <singh.gurjeet@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>