DDvO contrib: private key protection using OpenSSL's PEM password callback

[DDvO]

This implements password-based protection for PEM encoded private keys using OpenSSL's PEM password callback function. The changes made essentially affect only the example applications.

estclient.c and estserver.c now implement an extra option, "--keypass", used to specify a password source for en-/decrypting the PEM file contents used to store the respective private keys. According to the simple password callback function I implemented/used, the password may be either typed in on-the fly on the console or passed on the command line. As the key encryption algorithm I decided to use AES-128-CBC:
#define EST_PRIVATE_KEY_ENC EVP_aes_128_cbc() // The key wrap algorithm optionally used to protect private keys

I had to extend the est_load_key function, as well as the PEM write function and the RSA/EC key generation functions, to make use of the password callback. I moved all these helper functions:

  char *ossl_generate_private_RSA_key (int key_size, pem_password_cb *cb);
  char *ossl_generate_private_EC_key (int curve_nid, pem_password_cb *cb);
  char *ossl_private_key_to_PEM (const EVP_PKEY* pkey, pem_password_cb *cb);
  EVP_PKEY *ossl_read_private_key (const char *key_file, pem_password_cb *cb);
  EVP_PKEY *ossl_load_private_key (const unsigned char *key, int key_len, int format, pem_password_cb *cb);
  #define ossl_load_private_key_PEM(key) ossl_load_private_key((unsigned char*)(key),strlen(key),EST_FORMAT_PEM, NULL)

to est_ossl_util.c (where their implementation makes most sense to me, while they could also be defined, e.g., in est.c, example/util/utils.c, or test/util/test_utils.c) and declared them in est.h, such that they can be used easily by all applications and unit tests.

BTW, I managed to avoid adaptations to the code of the unit tests simply by employing the following macros:

  #define est_load_key(key, key_len, format) ossl_load_private_key(key,key_len,format, NULL)
  #define read_private_key(key_file) ossl_read_private_key(key_file, NULL)

The only action needed for the unit tests is to fully re-compile them once, which may be triggered simply by a "make clean".