conf: pass -- in a block command to the program

A command that takes -- as an argument lost it, and everything after
it, to the description:

    service name {
        command     = "cmd -- arg"
        description = "a cmd"
    }

ran `cmd` with description `arg`.

The block translator builds a legacy one-liner, appending the
description as "-- text", and service_register() splits that line at
the first "-- ", here the one in the command.

Split the description off only for legacy one-liners.  Blocks now hand
it to the new service_register_desc() as an argument.

Fixes #512

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/ChangeLog.md

@@ -153,6 +153,9 @@ All relevant changes are documented in this file.
 
 ### Fixes
 
+- Fix #512: `--` in the `command` of a block-format .conf, it was taken as the
+  start of the description, so `command = "cmd -- arg"` ran `cmd` with
+  description `arg`.  Found by Alexandru Paniș
 - Reject a .conf file that declares the same block title twice.  The title is
   the service identity and libConfuse merges two sections sharing one, so the
   file loaded as a single service holding a mix of both declarations,