Fix the SIGHUP signal race. ed's "event loop" operates a getchar(); check

Fix the SIGHUP signal race.  ed's "event loop" operates a getchar(); check
the hup flag before and after that call, when the buffer structures are stable
for write_file() to work.  Remove the hup handling from the SPL0() macro,
because this is run in at least one place during structure instability.
The SIGINT handler, which uses siglongjmp(), is also trusting the SPL1/SPL0
dance more than it should.
ok millert