More signal-handler cleanup.

More signal-handler cleanup.

* configure.ac (FLOAT_CHECK_DOMAIN): Comment fix (Bug#12327).
* src/floatfns.c: Comment fix.
* src/lisp.h (force_auto_save_soon): Declare regardless of SIGDANGER.
SIGDANGER might not be in scope so "#ifdef SIGDANGER" is not right,
and anyway the declaration is harmless even if SIGDANGER is not defined.
* src/syssignal.h (SIGIO): Also #undef if (! defined FIONREAD ||
defined BROKEN_FIONREAD).  systty.h formerly did this, but other
source files not surprisingly expected syssignal.h to define, or
not define, SIGIO, and it's cleaner to do it that way, for consistency.
Include <sys/ioctl.h>, for FIONREAD.
* src/systty.h (SIGIO): Do not #undef here; it's now syssignal.h's job.
This eliminates a problem whereby other files mysteriously had
to include "syssignal.h" before including "systty.h" if they
wanted to use "#ifdef SIGIO".