dir(): Do not assert(), return -1 and errno

The dir() function, although useless compared to glob(), called assert()
and perror() instead of returning an error code.

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

README.md

@@ -155,7 +155,8 @@ Generic Functions
 
   Wrapper for `scandir()` with optional filter.  Returns a list of
   names: files and directories that must be freed after use.  See
-  the unit test at the bottom for an example.
+  the unit test for an example, or take a look at `glob(3)`, it's
+  probably what you want anyway.
 
 - `fcopyfile(src, dst)`