Better handling of temp_dir misconfiguration

[alecpl]

Recent changes to attachments plugin(s) revealed an issue related to open_basedir. When open_basedir restriction is set and configured Roundcube temp_dir is not allowed by open_basedir. tempnam() will fallback to system temp directory (e.g. /tmp). In such a case path verification (in attachments plugin) will fail.

A proposed solution is to allow, aside of configured temp_dir, the system temp directory (result of sys_get_temp_dir()) in verify_path(). Optionally we could log a warning when we see that system temp directory is in use and it's different than the configured for Roundcube.

[DavidAnderson684]

In my case, after an update, the configured temp directory was not writeable. open_basedir was not in use. I think that "can't write to the temp_dir for any reason" is the more general issue, rather than just because of open_basedir.

[alecpl]

You're right. Done.