blocking wgetch() returns ERR and errno is set to EIO (input/output
error); if errno is set to something else, recover properly; this fixes
problems with nano's erroneously hanging up while e.g. resizing or
unsuspending in a chroot


git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3038 35c25a1d-7b9e-4130-9fde-d3aeb78583b8