Handling a SIGWINCH (which signals a change in window size) not when it

Handling a SIGWINCH (which signals a change in window size) not when it
happens but only when checking for keyboard input.  It now reports the
SIGWINCH via a special key value to the calling routine, to allow not
only the main editor but also the help viewer and the file browser to
adapt their display to the new size.
Patch by Mahyar Abbaspour, somewhat edited by Benno.

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@5228 35c25a1d-7b9e-4130-9fde-d3aeb78583b8