ft: helper function for accessing credentials directory

This adds new function `systemd:credentials/1` which returns path for
credentials' files. This function checks if the `$CREDENTIALS_DIRECTORY`
variable was set and if it was set, then the path is always returned. It
is left up to the user to check if the file is existing and/or correct
one.

Close #41

src/systemd.erl

@@ -59,7 +59,8 @@
     store_fds/1,
     clear_fds/1,
     booted/0,
-    is_journal/1
+    is_journal/1,
+    credentials/1
 ]).
 
 -export([notify_spawn/2]).
@@ -403,6 +404,25 @@ file_info(_) ->
 
 %% ----------------------------------------------------------------------------
 
+%% @doc
+%% Returns path for credentials file.
+%%
+%% Returns `error' when there is no `$CREDENTIALS_DIRECTORY' environment
+%% variable, `{ok, Path}' otherwise.
+%%
+%% For details see [https://systemd.io/CREDENTIALS/]
+%%
+%% @since 0.7.0
+%% @end
+credentials(Name) ->
+    case os:getenv("CREDENTIALS_DIRECTORY") of
+        false -> error;
+        Dir ->
+            {ok, filename:join(Dir, Name)}
+    end.
+
+%% ----------------------------------------------------------------------------
+
 %% @doc
 %% Returns list of file descriptors passed to the application by systemd.
 %%