Extract public quit!/disconnect from do_finish

This renames `#do_finish` to `#quit!`, makes it public, adds an option
to convert exceptions into warnings, and extracts the contents of the
ensure block into a new public `#disconnect` method.  The motivation for
making these public is given in the rdoc.

As documented in the rdoc, `#quit!`:
> Calls #quit and ensures that #disconnect is called.  Returns the
> result from #quit.  Returns +nil+ when the client is already
> disconnected or when a prior error prevents the client from calling
> #quit.  Unlike #finish, this an exception will not be raised when the
> client has not started.
>
> When <tt>exception: :warn</tt> is specified, when #quit raises a
> StandardError, a warning will be printed and the exception is
> returned, not re-raised.  When <tt>exception: false</tt> is specified,
> a warning will not be printed.  This is useful when the connection
> must be dropped, for example in a test suite or due to security
> concerns.

As documented in the rdoc, `#disconnect`:
> Disconnects the socket without checking if the connection has started
> yet, and without sending a final QUIT message to the server.
>
> Generally, either #finish or #quit! should be used instead.

lib/net/smtp.rb

@@ -642,7 +642,7 @@ def start(*args, helo: nil, user: nil, secret: nil, password: nil, authtype: nil
           do_start helo, user, secret, authtype
           return yield(self)
         ensure
-          do_finish
+          quit!
         end
       else
         do_start helo, user, secret, authtype
@@ -654,7 +654,7 @@ def start(*args, helo: nil, user: nil, secret: nil, password: nil, authtype: nil
     # Raises IOError if not started.
     def finish
       raise IOError, 'not yet started' unless started?
-      do_finish
+      quit!
     end
 
     private
@@ -728,15 +728,46 @@ def do_helo(helo_domain)
       raise
     end
 
-    def do_finish
+    public
+
+    # Calls #quit and ensures that #disconnect is called.  Returns the result
+    # from #quit.  Returns +nil+ when the client is already disconnected or when
+    # a prior error prevents the client from calling #quit.  Unlike #finish,
+    # this an exception will not be raised when the client has not started.
+    #
+    # When <tt>exception: :warn</tt> is specified, when #quit raises a
+    # StandardError, a warning will be printed and the exception is returned,
+    # not re-raised.  When <tt>exception: false</tt> is specified, a warning
+    # will not be printed.  This is useful when the connection must be dropped,
+    # for example in a test suite or due to security concerns.
+    #
+    # Related: #finish, #quit, #disconnect
+    def quit!(exception: true)
       quit if @socket and not @socket.closed? and not @error_occurred
+    rescue => ex
+      if exception == :warn
+        warn "%s during %p #%s: %s" % [ex.class, self, __method__, ex]
+      elsif exception
+        raise ex
+      end
+      ex
     ensure
+      disconnect
+    end
+
+    # Disconnects the socket without checking if the connection has started yet,
+    # and without sending a final QUIT message to the server.
+    #
+    # Generally, either #finish or #quit! should be used instead.
+    def disconnect
       @started = false
       @error_occurred = false
       @socket.close if @socket
       @socket = nil
     end
 
+    private
+
     def requires_smtputf8(address)
       if address.kind_of? Address
         !address.address.ascii_only?