TLS: refactor cert=/key= storage in libsecurity

TLS: refactor cert=/key= storage in libsecurity

This updates the cert=/key= filename storage from single entries
in PeerOptions to a list of key pairs in preparation for supporting
multiple certificates on client or server TLS contexts.

key= following a cert= parameter is now enforced, rather than just
warned about.

squid.conf can now be configured with multiple [cert= [key=...]]
pairs of filenames, however only the first is used. This differs
from older behaviour where the last value(s) were used. But since
configurations with multiple values was not supported previously
this seems acceptible breakage.

Since the multi-cert support is not fully existing yet this config
ability is left undocumented for now.