Better fail with E_EXIST if rename(3) fails

...it leads to better handling of the failure downstream, if e.g. this is an
attempt to copy a file from the presets to an existing file of a previously
partly generated UserInstallation, and the presets and the UserInstallation are
on different drives and rename(3) fails with EXDEV.  (Without this fix, removing
the UserInstallation's registrymodifications.xcu caused restarting soffice to
fail with "User installation could not be completed" in this case; regression
introduced with 6edbcc1b "cid#1242936 handle
rename failure.")

Change-Id: I409698c4e2e6cc6ccd8b2be80a393c9657b5125d
(cherry picked from commit d2a7abeb)
Reviewed-on: https://gerrit.libreoffice.org/18898Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

sal/osl/unx/file_misc.cxx

@@ -816,7 +816,7 @@ static oslFileError oslDoCopy(const sal_Char* pszSourceFileName, const sal_Char*
                     "sal.osl",
                     "rename(" << pszDestFileName << ", " << tmpDestFile
                         << ") failed with errno " << e);
-                return osl_File_E_BUSY; // for want of a better error code
+                return osl_File_E_EXIST; // for want of a better error code
             }
         }
     }