net/unison: Update to 2.52.1

   * Deprecate ocaml < 4.08: 2.53.0 will reqire 4.08 or higher.
   * Add man page
   * Remove rsh:// URI scheme (ssh:// of course remains).
   * Significant bugfixes and minor improvements
   * Deprecate "backups" preference (see manual for alternatives)
   * Deprecate "stream" and "halfduplex" preferences
