Modernize: use sigaction() whenever possible. Remove uses of common.

bsd_signal() may not be defined, and there is really no reason to even
try to use it if sigaction() is avaiable; using sigaction() guarantees
the semantics we really want.

Replace uses of common variables with explicit instantiation and
extern declarations in a header file.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

lib/Makefile

@@ -25,5 +25,3 @@ libxtra.a: $(LIBOBJS)
 	-rm -f libxtra.a
 	$(AR) libxtra.a $(LIBOBJS)
 	$(RANLIB) libxtra.a
-
-