Migrate to POSIX signals. (#190)

This removes SYSVSIGNALS as we're always and only using POSIX
signals now.

Some platform differences have been papered over. We used to
only ignore SIGPIPE when using BSD signals, but we now ignore
it all the time.

While the SIGFPE code will now compile, it hasn't been updated
to work on modern OSes fully yet as it will need to enable
the correct FP exceptions.

src/ether.c

@@ -366,29 +366,28 @@ LispPTR ether_get(LispPTR args[])
   LispPTR result = NIL;
 #ifndef NOETHER
   LispPTR MaxByteCount;
-#ifndef SYSVSIGNALS
-  int interrupt_mask;
-#endif
+  sigset_t signals;
+
   MaxByteCount = 2 * (0xFFFF & args[0]); /* words to bytes */
 
   DBPRINT(("Ether Get.  "));
 
-#ifdef SYSVSIGNALS
-  sighold(SIGIO);
-#else
-  interrupt_mask = sigblock(sigmask(SIGIO)); /* turn off ENET interrupts */
-#endif /* SYSVSIGNALS */
+  sigemptyset(&signals);
+  sigaddset(&signals, SIGIO);
+
+  /* turn off ENET interrupts */
+  sigprocmask(SIG_BLOCK, &signals, NULL);
+
   if (ether_fd > 0 && (MaxByteCount > 0)) {
     ether_buf = (u_char *)Addr68k_from_LADDR(args[1]);
     ether_bsize = MaxByteCount; /* do this LAST; it enables reads */
     result = get_packet();
     /*	check_ether(); for old behavior, move comment to above line */
   }
-#ifdef SYSVSIGNALS
-  sigrelse(SIGIO);
-#else
-  sigsetmask(interrupt_mask);                /* interrupts back on */
-#endif /* SYSVISGNALS */
+
+  /* enable interrupts */
+  sigprocmask(SIG_UNBLOCK, &signals, NULL);
+
 #endif /* NOETHER */
 
   return (result);