Make dc_get_info() work on a closed context

There is very little API guarantees about this, but clients seem to
expect *something* to work on a closed context.  So split this up into
static info an more dynamic context-related info, but let's not
provide any guarantees about what keys are available when.

Fixes #599

deltachat-ffi/deltachat.h

@@ -405,9 +405,14 @@ char*           dc_get_config                (dc_context_t* context, const char*
 
 /**
  * Get information about the context.
+ *
  * The information is returned by a multi-line string
  * and contains information about the current configuration.
  *
+ * If the context is not open or configured only a subset of the information
+ * will be available.  There is no guarantee about which information will be
+ * included when however.
+ *
  * @memberof dc_context_t
  * @param context The context as created by dc_context_new().
  * @return String which must be free()'d after usage.  Never returns NULL.

deltachat-ffi/src/lib.rs

@@ -347,9 +347,12 @@ pub unsafe extern "C" fn dc_get_info(context: *mut dc_context_t) -> *mut libc::c
         return dc_strdup(ptr::null());
     }
     let ffi_context = &*context;
-    ffi_context
-        .with_inner(|ctx| render_info(ctx.get_info()).unwrap_or_default().strdup())
-        .unwrap_or_else(|_| "".strdup())
+    let guard = ffi_context.inner.read().unwrap();
+    let info = match guard.as_ref() {
+        Some(ref ctx) => ctx.get_info(),
+        None => context::get_info(),
+    };
+    render_info(info).unwrap_or_default().strdup()
 }
 
 fn render_info(