Socket: adds closeOnDestruction flag to ctor arguments

[denizzzka]

Rationale

There is a case that is not covered by the std.socket.Socket:
When we get a ready-made socket_t from somewhere (from a 3rd-party library, for example), using it idiomatically in the D style, and after it we should just leave the socket "as is", because all other handling of this socket_t is performed by 3rd-party code.

Currently, if we want to do it this way, we have to duplicate the socket:
https://github.com/denizzzka/dpq2/blob/master/src/dpq2/connection.d#L227
and then Socket will close duplicate in desctructor without side-effects.

Solution

closeOnDestruction flag added into ctor arguments, it allows not close underlying system socket in the destructor of Socket

Pre-review checklist

• I have performed a self-review of my code.
• If my PR fixes a bug or introduces a new feature, I have added thorough tests.

I think this case doesn't need testing.

• If my changes are non-trivial and do not concern a reported issue, I have added a changelog entry.

[rikkimax]

When I said flag I meant: https://dlang.org/phobos/std_typecons.html#Flag

I'll leave it to others to decide if this is a requirement or not.

[denizzzka]

A workaround for this problem was devised: #11040 (comment)