Bug 4135: Support \-escaped character in regex patterns

Bug 4135: Support \-escaped character in regex patterns

Squid cannot parse regex patterns as quoted strings since the pattern may
itself contain quote characters as part of the syntax.

Since we updated the squid.conf ConfigParser it is now possible to
handle regex patterns containing quoted-pair (\-escaped) characters
properly.

Add support for escaping by detecting the '\' characters as token
delimiters, and explicitly skipping the following character regardless
of whether it is a SP or not.

Escape detection is only added during parsing of regex tokens or
files listing regex patterns.