Use negative_errno() to assert errno is positive after a few system calls

This is not particularly intrusive because it happens in simple
utility functions. It helps gcc understand that error codes
are negative.

This gets a rid of most of the remaining warnings.

src/basic/path-util.c

@@ -102,7 +102,7 @@ int path_make_absolute_cwd(const char *p, char **ret) {
 
                 cwd = get_current_dir_name();
                 if (!cwd)
-                        return -errno;
+                        return negative_errno();
 
                 c = strjoin(cwd, "/", p, NULL);
         }