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/fileio.c

@@ -773,7 +773,8 @@ static int merge_env_file_push(
 
         assert(env);
 
-        expanded_value = replace_env(value, *env, REPLACE_ENV_USE_ENVIRONMENT);
+        expanded_value = replace_env(value, *env,
+                                     REPLACE_ENV_USE_ENVIRONMENT|REPLACE_ENV_ALLOW_BRACELESS);
         if (!expanded_value)
                 return -ENOMEM;
 
@@ -787,6 +788,10 @@ int merge_env_file(
                 FILE *f,
                 const char *fname) {
 
+        /* NOTE: this function supports braceful and braceless variable expansions,
+         * unlike other exported parsing functions.
+         */
+
         return parse_env_file_internal(f, fname, NEWLINE, merge_env_file_push, env, NULL);
 }