Moved regcomp(3)-specific RegexPattern code inside RegexPattern (#972)

Moved regcomp(3)-specific RegexPattern code inside RegexPattern (#972)

Also moved non-ACL regex configuration parsing code inside ConfigParser.
It is possible to move ACL (data) regex configuration parsing code as
well, but that move is a lot more complex due to regex-pattern-joining
optimizations, and there are no immediate plans to support
non-regcomp(3) regexes in ACL data. We may do that move later as we get
more experience with non-regcomp(3) regexes and decide to join them too.

These moves clean up existing regex-using code and allow adding support
for non-regcomp(3) regexes (e.g., regexes based on C++11 <regex>)
without adjusting ConfigParser::regex() callers. Such support would also
require "virtualizing" RegexPattern. To avoid increasing complexity and
hurting performance, that (simpler) step should be done only if we
decide to actually add support for non-regcomp(3) regexes.

The above changes allowed us to improve RegexData error reporting: Squid
no longer reports REs _optimization_ failure when it is an individual RE
that is broken (and reported as such). Squid still ignores the fact that
broken REs can be "optimized" into a completely "different" valid
combined RE: We do not compile individual REs unless optimization fails.

Also simplified and polished affected code.