git-instaweb: Wait for server to start before running web browser

git-instaweb: Wait for server to start before running web browser

Add generic httpd_is_ready subroutine, which busy-waits for web server to
be started, by checking if $port is opened on localhost.  This is used to
avoid situation where web browser is started before web server is ready to
accept connection, and fails.

It uses IO::Socket::INET module, which is core Perl module since v5.6.0.

Alternate solution, possible for those web servers that can run arbitrary
code hooks after they bind the listen socket (after they start accepting
connections), would be to use some kind of blocking mechanism: FIFO or
lockfile, see
  http://thread.gmane.org/gmane.comp.version-control.git/147337/focus=147566

This can be always added later, as a web server specific branch in
httpd_is_ready function.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>