in get_key_buffer(), if we fail to get a character over MAX_BUF_SIZE

in get_key_buffer(), if we fail to get a character over MAX_BUF_SIZE
times in a row, hang up regardless of the value of errno; this fixes a
problem where nano doesn't terminate properly under xterm if the user
su's to root, runs nano, and then closes the terminal window

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3309 35c25a1d-7b9e-4130-9fde-d3aeb78583b8