Avoid reading tls-cert=bundle twice when loading certificates (#1073)

Avoid reading tls-cert=bundle twice when loading certificates (#1073)

After opening and reading the certificate bundle to load the required
traffic-signing certificate, Squid was opening and reading the same file
again to load the optional intermediate certificates. Loading the
traffic-signing certificate twice triggered bugs and further
complicating naturally tricky code.

No functionality changes expected except minor cache.log message changes
and certificate reporting improvements in "squid -k parse" mode.