remove.c: accommodate systems with negative errno values

remove.c: accommodate systems with negative errno values

This is required at least on Haiku and BeOS.
* src/remove.c (write_protected_non_symlink): Return 1 for a write-
protected non-symlink, 0 if we determine it's not, and -1 upon
error (setting errno accordingly only in this final case).
(prompt): Deal with the changed semantics of the above function.
Based on this patch from Axel Dörfler:
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/13071