easy: allow connect-only handle reuse with easy_perform

- Detach and disconnect an attached connection before performing.

Prior to this change it was not possible to safely reuse an easy handle
with an attached connection in a second call to curl_easy_perform. The
only known case of this is a connect-only type handle where the
connection was detached when curl_easy_perform returned, only to be
reattached by either curl_easy_send/recv.

This commit effectively reverts 2f8ecd5d and be82a360, the latter of
which treated the reuse as an error. Prior to that change undefined
behavior may occur in such a case.

Bug: https://curl.se/mail/lib-2025-01/0044.html
Reported-by: Aleksander Mazur

Closes https://github.com/curl/curl/pull/16008

tests/data/test696

@@ -7,7 +7,7 @@ HTTP GET
 </info>
 
 <reply>
-<data>
+<data nocheck="yes">
 HTTP/1.1 200 OK swsclose
 Server: test-server/fake
 Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
@@ -38,15 +38,33 @@ http://%HOSTIP:%HTTPPORT
 #
 # Verify data after the test has been "shot"
 <verify>
+<stdout>
+HTTP/1.1 200 OK swsclose
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+Content-Length: 6
+Connection: close
+
+-foo-
+HTTP/1.1 200 OK swsclose
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+Content-Length: 6
+Connection: close
+
+-foo-
+</stdout>
 <protocol>
 GET /556 HTTP/1.1
 Host: ninja
 
+GET /556 HTTP/1.1
+Host: ninja
+
 </protocol>
 
-# 43 == CURLE_BAD_FUNCTION_ARGUMENT
 <errorcode>
-43
+0
 </errorcode>
 </verify>
 </testcase>