lib-git-svn.sh: Move web-server handling code into separate function

lib-git-svn.sh: Move web-server handling code into separate function

This library file is currently sourced by 57 test files, of which
only four may (optionally) start a web-server in order to access
the svn repo via an http url, rather than a file url.

In addition to isolating the current web-server handling code from
the majority of tests, in a new prepare_httpd function, we also
add some more error checking and reporting code to validate the
apache installation. Only those tests which attempt to start the
web-server, by calling start_httpd, will execute this code.

Note that it is important for start_httpd to return an error
indication, if prepare_httpd fails, so that the failure to use
the web-server, as requested by the user, should not go unnoticed.
(Unless the svnrepo variable is set to an http url at the end of
start_httpd, the remaining tests will use file urls, without
comment.)

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>