Detect HTTP header ACL issues

Detect HTTP header ACL issues

rep_header and req_header ACL types cannot match multiple different
headers in one test (unlike Squid-2 appears to have done). Produce
an ERROR and ignore the extra line(s) instead of silently changing
all the previous regex to match the second header name.

Also detect and ERROR when header name is missing entirely. Ignore
these lines instead of asserting.