fixed `tests/authorize.sh' assertions

A change in the error message for wrong shared secrets broke half of the
`authorize.sh' tests. I've introduced a new `test_it_first_line_fail'
function to handle multi-line response bodies that only checks the first
line (on top of the exit status) as the name implies.
open the secret file immediately for the -S option

The varnish-agent will usually be run as a daemon, executed by a systemd
or upstart or sysv or <put your favourite system here> service as root.
The problem is that varnish-agent will attempt to open the secret file
after giving up its privileges (setgid and setuid) and won't be able to
open a secret file with a 400 or 600 mode that should be owned by root.

If the -S option was not found on the command line, the varnish agent
will search for it in the shared memory log. Assuming varnishd is
running on the same machine, varnish-agent is very likely to have a read
access to the shmlog.
s/AM_CONFIG_HEADER/AC_CONFIG_HEADERS/

According to rpmlint(1), AM_CONFIG_HEADER is deprecated.
make the rpm own the agent_secret file