2.16 - to rebase: New password-based authenticated encryption sample program pbcrypt

[gilles-peskine-arm]

This PR introduces programs/cipher/pbcrypt, a new sample program to demonstrate password-based authenticated encryption.

This is meant to illustrate good practice (with the limitation of using PBKDF2 rather than something more modern like Argon2), so please review accordingly. A few caveats:

• The program works in a specific way: streaming AEAD. Since the library doesn't support streaming CCM, the program doesn't support it.
• The use of PKCS#12 key derivation is hard-coded because the library doesn't have a generic interface to password-based key derivation.
• The program should build in all configurations for which the library builds, but it may not run usefully in some “exotic” configurations (e.g. GCM enabled but no block cipher that supports GCM).
• The program relies on argv and stdio. It does password-based key derivation so it isn't meant for microcontrollers anyway.

Provide a demo usage script and run it from all.sh.

Remove the badly designed aescrypt2. Fix #1906.

This PR is for 2.16. I'll forward-port it to other branches once it's approved.

• Add ChachaPoly to the #ifdef about main and to cipher_is_aead.
• Replace mbedtls_zeroize by mbedtls_platform_zeroize.

[mpg]

This needs to be rebased on one of the active branches now that 2.16 is EOL.

[tom-daubney-arm]

This PR will now be closed. The aescrypt2 program has already been removed from the library and there is a PR currently in progress (#5064) that will add a good selection of example programs.

[gilles-peskine-arm]

The PSA crypto API example programs are not relevant. There's nothing else in the works that's like pbcrypt — showing how to build a relatively simple but still decent PBKDF algorithm out of the mbedtls API.

That being said, we've been putting no effort in example programs for years and I don't see thing changing any time soon. So ok, let's just forget about this one.