As a first step towards safe signal handling, improve the h_int()

As a first step towards safe signal handling, improve the h_int()
and h_winch() signal handlers to make one single store to a
sig_atomic_t variable.  Note that the h_hup() and h_term() signal
handlers are still unsafe after this commit because they also set
the "killersig" (how fitting!) field in a global struct.

Despite storing information in static global variables rather than
in structs passed around as arguments, this patch does not cause a
change in behaviour because there is always exactly one GS object,
initialized using gs_init() called from the top of main(), and
screen_init() stores a pointer to this one and only GS object in
the .gp member of each and every SCR object.  Talk about useless
abstraction...

Problem pointed out by deraadt@.
Patch from Tim <trondd at kagu hyphen tsuchi dot com> on tech@.
OK deraadt@.