Allow braceless variables to be expanded

(Only in environment.d files.)

We have only basic compatibility with shell syntax, but specifying variables
without using braces is probably more common, and I think a lot of people would
be surprised if this didn't work.

src/basic/env-util.h

@@ -31,6 +31,7 @@ bool env_assignment_is_valid(const char *e);
 
 enum {
         REPLACE_ENV_USE_ENVIRONMENT = 1u,
+        REPLACE_ENV_ALLOW_BRACELESS = 2u,
 };
 
 char *replace_env(const char *format, char **env, unsigned flags);