Bug 2833 pt2: Collapse internal revalidation requests (SMP-unaware caches), again.

Bug 2833 pt2: Collapse internal revalidation requests (SMP-unaware caches), again.

The security fix in v5 r14979 had a negative effect on collapsed
forwarding. All "private" entries were considered automatically
non-shareable among collapsed clients. However this is not true: there
are many situations when collapsed forwarding should work despite of
"private" entry status: 304/5xx responses are good examples of that.
This patch fixes that by means of a new StoreEntry::shareableWhenPrivate
flag.

The suggested fix is not complete: To cover all possible situations, we
need to decide whether StoreEntry::shareableWhenPrivate is true or not
for all contexts where StoreEntry::setPrivateKey() is used. This patch
fixes only few important cases inside http.cc, making CF (as well
collapsed revalidation) work for some [non-cacheable] response status
codes, including 3xx, 5xx and some others.

The original support for internal revalidation requests collapsing
was in trink r14755 and referred to Squid bugs 2833, 4311, and 4471.