calculate OPACBaseURL instead of using syspref

calculate OPACBaseURL instead of using syspref

Calculate OPACBaseURL from the
CGI environment variables instead of using the
system preference.  As a result of this patch,
it should be possible to change the hostname,
protocol, or port of the OPAC without having to
reset a system preference.

Also added a FIXME to opac/unapi - the URL of
the SRU/W server has no necessary relationship
to the URL of the OPAC.

Once this patch is confirmed, the syspref can
be removed.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>