Avoid writing S/MIME plaintext to disk entirely

[fabacab]

Two options come to mind:

• Using /dev/shm/$some_path if the plugin is running in a GNU/Linux environment with a mounted tmpfs as the S/MIME $infile. This would not be portable to, for example, Windows-based servers. We could auto-detect our environment and simply perform the optimization if possible, falling back to the existing implementation if it is not.
• Using php://memory or php://temp streams and manually constructing the PKCS#7 formatted S/MIME message from strings instead of using openssl_pkcs7_encrypt(), since that function requires file paths. This would be more work but will also be more portable across operating systems.

[p7996619]

I actually have a branch (7f69e8d) waiting regarding "secure" deletion :)
Also looked into streams and I think that would be the best approach in the long run, but seems like much work.

I'll submit a PR if you think it's ready for it

[fabacab]

I'll submit a PR if you think it's ready for it

@GithubuserX Sure!

[DanielRuf]

• php://memory

Is a good idea. I've done two small experiments in the past with a few small benchmarks.
https://github.com/DanielRuf/inmemory-benchmarks