directory: converted isRootDirectory() to an inline function

The function isRootDirectory() is tiny and can be converted to an
inline function.  Don't allow name==NULL.

src/directory.h

@@ -46,7 +46,11 @@ void directory_init(void);
 
 void directory_finish(void);
 
-int isRootDirectory(const char *name);
+static inline bool
+isRootDirectory(const char *name)
+{
+	return name[0] == 0 || (name[0] == '/' && name[1] == 0);
+}
 
 struct directory *
 directory_get_root(void);