kill: allow kill -0 for testing process existence.

Allow num_to_sig(0) so kill doesn't disallow -0 because it can't
round-trip, and then teach list_signals() to avoid the special case of
signal 0 rather than assume that all the signals it shouldn't print
don't have names now signal 0 has the name "0".

lib/lib.c

@@ -931,7 +931,7 @@ void sigatexit(void *handler)
 // Output a nicely formatted table of all the signals.
 void list_signals(void)
 {
-  int i = 0, count = 0;
+  int i = 1, count = 0;
   unsigned cols = 80;
   char *name;
 

lib/portability.c

@@ -422,6 +422,7 @@ int posix_fallocate(int fd, off_t offset, off_t length)
 #define SIGNIFY(x) {SIG##x, #x}
 
 static const struct signame signames[] = {
+  {0, "0"},
   // POSIX
   SIGNIFY(ABRT), SIGNIFY(ALRM), SIGNIFY(BUS),
   SIGNIFY(FPE), SIGNIFY(HUP), SIGNIFY(ILL), SIGNIFY(INT), SIGNIFY(KILL),