Merge topic branch t/openssl-3 into pu

Started on 2023-04-29.

The series starts with a few cleanups and crypto-backend tweaks to
beat the openssl specific code into shape for the main objective:
the switch of the RSA encryption and decryption routines to the
high-level EVP API. This has become necessary because the old RSA
API functions have been deprecated in openssl-3.

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- The openssl RSA code has been converted to EVP and no longer uses
  any deprecated openssl API functions.

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* refs/heads/t/openssl-3:
  openssl: Reactivate openssl warnings.
  openssl: Use the EVP library for RSA private decryption.
  openssl: Use the EVP library for RSA public encryption.
  apc_priv_decrypt: Let the callee allocate the buffer.
  apc_pub_encrypt: Let the callee allocate the buffer.
  openssl: Assume that openssl allocation functions functions succeed.
  openssl: Introduce openssl_perror().
  openssl: Don't pass pointers to RSA structures around.
  openssl: Kill rsa coefficient computations.
  client: Check buffer size returned by apc_priv_decrypt().

# Conflicts:
#	configure.ac