Add config yadm.openssl-old

The newer versions (OpenSSL 1.1.1 or LibreSSL 2.9.1) support the pbkdf2
key derivation function, while older versions do not. In addition the
new versions have changed the default digest to SHA256 instead of MD5.

Files encrypted with older versions would throw warnings about
deprecated key derivation used files encrypted with newer versions +
pbkdf2 would not be decryptable using older versions These problems
matter, when many users maintain their dotfiles across different systems
with different levels of OpenSSL support.

A new boolean config option has been added, yadm.openssl-old

* If false, use options -pbkdf2 -iter 100000 -md sha512
* If true, use options -md md5 (and if decrypting with newer versions
  warnings will be printed)

test/conftest.py

@@ -124,6 +124,7 @@ def supported_configs():
         'yadm.gpg-program',
         'yadm.gpg-recipient',
         'yadm.openssl-ciphername',
+        'yadm.openssl-old',
         'yadm.openssl-program',
         'yadm.ssh-perms',
         ]