Allow HTTP tests to run on Darwin

Allow HTTP tests to run on Darwin

This patch allows the HTTP tests to run on OS X 10.5. It is not
sufficient to be able to pass in LIB_HTTPD_PATH and
LIB_HTTPD_MODULE_PATH alone, as the apache.conf also needs a couple
tweaks.

These changes are put into an <IfDefine> to keep them Darwin specific,
but this means lib-httpd.sh needs to be modified to pass -DDarwin to
apache when running on Darwin. As long as we're making this change to
lib-httpd.sh, we may as well set LIB_HTTPD_PATH and
LIB_HTTPD_MODULE_PATH to appropriate default values for the platform.

Note that we now pass HTTPD_PARA to apache at shutdown as well.
Otherwise apache will emit a harmless, but noisy warning that LogFormat
is an unknown directive.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>