org-mobile: Specify message digest for OpenSSL commands

* lisp/org-mobile.el (org-mobile-encrypt-file):
(org-mobile-decrypt-file): Fix the message digest as MD5 for
compatibility across OpenSSL versions.

OpenSSL switched to using SHA256 by default for symmetric encryption
in version 1.1.  Unfortunately that means that newer versions of the
openssl command line tool can not decrypt encrypted org-mobile files
without the '-md md5' option.  This commit changes the shell commands
in org-mobile-encrypt-file and org-mobile-decrypt-file to explicitly
specify MD5 as the hash digest.

TINYCHANGE

lisp/org-mobile.el

@@ -693,7 +693,7 @@ encryption program does not understand them."
 (defun org-mobile-encrypt-file (infile outfile)
   "Encrypt INFILE to OUTFILE, using `org-mobile-encryption-password'."
   (shell-command
-   (format "openssl enc -aes-256-cbc -salt -pass %s -in %s -out %s"
+   (format "openssl enc -md md5 -aes-256-cbc -salt -pass %s -in %s -out %s"
 	   (shell-quote-argument (concat "pass:"
 					 (org-mobile-encryption-password)))
 	   (shell-quote-argument (expand-file-name infile))
@@ -702,7 +702,7 @@ encryption program does not understand them."
 (defun org-mobile-decrypt-file (infile outfile)
   "Decrypt INFILE to OUTFILE, using `org-mobile-encryption-password'."
   (shell-command
-   (format "openssl enc -d -aes-256-cbc -salt -pass %s -in %s -out %s"
+   (format "openssl enc -md md5 -d -aes-256-cbc -salt -pass %s -in %s -out %s"
 	   (shell-quote-argument (concat "pass:"
 					 (org-mobile-encryption-password)))
 	   (shell-quote-argument (expand-file-name infile))