Now the non-SSH backends critically depend on it, it's important not
to forget to send it, for any socket type that's going to be used for
any of those backends. But ProxySocket, and the Unix and Windows
'socket' types wrapping pipes to local subprocesses, were not doing
so.
Some of these socket types don't have a SockAddr available to
represent the destination host. (Sometimes the concept isn't even
meaningful). Therefore, I've also expanded the semantics of
PLUGLOG_CONNECT_SUCCESS so that the addr parameter is allowed to be
NULL, and invented a noncommittal fallback version of the log message
in that situation.
The call to plug_closing very likely destroys the FdSocket entirely,
so we shouldn't wait until after that to clean up its input fd via
lots of dereferences.
This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.
